diff --git a/python/_project.pyx b/python/_project.pyx
index a8ff9dc..4f69953 100644
--- a/python/_project.pyx
+++ b/python/_project.pyx
@@ -23,7 +23,7 @@ cdef extern from "project_tool.hpp" namespace "":
 @cython.cdivision(True)
 cdef DTYPE_t interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
                                         DTYPE_t z, 
-                                        npx.ndarray[DTYPE_t, ndim=3] d, DTYPE_t Lbox) except? 0:
+                                        DTYPE_t[:,:,:] d, DTYPE_t Lbox) nogil except? 0:
 
     cdef int Ngrid = d.shape[0]
     cdef DTYPE_t inv_delta = Ngrid/Lbox
@@ -60,9 +60,15 @@ cdef DTYPE_t interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y,
     iy = iy%Ngrid
     iz = iz%Ngrid
 
-    assert ((ix >= 0) and ((jx) < Ngrid))
-    assert ((iy >= 0) and ((jy) < Ngrid))
-    assert ((iz >= 0) and ((jz) < Ngrid))
+    if (ix < 0)  or (jx >= Ngrid):
+      with gil:
+        assert ((ix >= 0) and ((jx) < Ngrid))
+    if (iy < 0)  or (jy >= Ngrid):
+      with gil:
+        assert ((iy >= 0) and ((jy) < Ngrid))
+    if (iz < 0)  or (jz >= Ngrid):
+      with gil:
+        assert ((iz >= 0) and ((jz) < Ngrid))
 
     f[0][0][0] = (1-rx)*(1-ry)*(1-rz)
     f[1][0][0] = (  rx)*(1-ry)*(1-rz)
@@ -89,7 +95,7 @@ cdef DTYPE_t interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y,
 @cython.cdivision(True)
 cdef DTYPE_t interp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
                                DTYPE_t z, 
-                               npx.ndarray[DTYPE_t, ndim=3] d, DTYPE_t Lbox) except? 0:
+                               DTYPE_t[:,:,:] d, DTYPE_t Lbox) nogil except? 0:
     
     cdef int Ngrid = d.shape[0]
     cdef DTYPE_t inv_delta = Ngrid/Lbox
@@ -110,11 +116,14 @@ cdef DTYPE_t interp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
     rz -= iz
 
     if ((ix < 0)  or (ix+1) >= Ngrid):
-      raise IndexError("X coord out of bound (ix=%d, x=%g)" % (ix,x))
+      with gil:
+        raise IndexError("X coord out of bound (ix=%d, x=%g)" % (ix,x))
     if ((iy < 0)  or (iy+1) >= Ngrid):
-      raise IndexError("Y coord out of bound (iy=%d, y=%g)" % (iy,y))
+      with gil:
+        raise IndexError("Y coord out of bound (iy=%d, y=%g)" % (iy,y))
     if ((iz < 0)  or (iz+1) >= Ngrid):
-      raise IndexError("Z coord out of bound (iz=%d, z=%g)" % (iz,z))
+      with gil:
+        raise IndexError("Z coord out of bound (iz=%d, z=%g)" % (iz,z))
     #   assert ((ix >= 0) and ((ix+1) < Ngrid))
 #    assert ((iy >= 0) and ((iy+1) < Ngrid))
 #    assert ((iz >= 0) and ((iz+1) < Ngrid))
@@ -139,6 +148,7 @@ cdef DTYPE_t interp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
         d[ix  ,iy+1,iz+1] * f[0][1][1] + \
         d[ix+1,iy+1,iz+1] * f[1][1][1]
 
+@cython.boundscheck(False)
 def interp3d(x not None, y not None, 
              z not None,
              npx.ndarray[DTYPE_t, ndim=3] d not None, DTYPE_t Lbox,
@@ -148,8 +158,13 @@ def interp3d(x not None, y not None,
     Compute the tri-linear interpolation of the given field (d) at the given position (x,y,z). It assumes that they are box-centered coordinates. So (x,y,z) == (0,0,0) is equivalent to the pixel at (Nx/2,Ny/2,Nz/2) with Nx,Ny,Nz = d.shape. If periodic is set, it assumes the box is periodic 
 """
     cdef npx.ndarray[DTYPE_t] out
-    cdef npx.ndarray[DTYPE_t] ax, ay, az
-    cdef int i
+    cdef DTYPE_t[:] out_slice
+    cdef DTYPE_t[:] ax, ay, az
+    cdef long i
+    cdef long Nelt
+    cdef int myperiodic
+
+    myperiodic = periodic
 
     if d.shape[0] != d.shape[1] or d.shape[0] != d.shape[2]:
       raise ValueError("Grid must have a cubic shape")
@@ -164,12 +179,15 @@ def interp3d(x not None, y not None,
         assert ax.size == ay.size and ax.size == az.size
         
         out = np.empty(x.shape, dtype=DTYPE)
-        if periodic:
-          for i in range(ax.size):
-            out[i] = interp3d_INTERNAL_periodic(ax[i], ay[i], az[i], d, Lbox)
-        else:
-          for i in range(ax.size):
-            out[i] = interp3d_INTERNAL(ax[i], ay[i], az[i], d, Lbox)
+        out_slice = out
+        Nelt = ax.size
+        with nogil:
+          if myperiodic:
+            for i in prange(Nelt):
+              out[i] = interp3d_INTERNAL_periodic(ax[i], ay[i], az[i], d, Lbox)
+          else:
+            for i in prange(Nelt):
+              out[i] = interp3d_INTERNAL(ax[i], ay[i], az[i], d, Lbox)
 
         return out
     else: