  New high resolution, high sensitivity WSRT HI synthesis observations of the
spiral galaxy NGC 3631 are presented. In the total atomic hydrogen map, the
spiral arms are well distinguished from the interarm regions, while the
sensitivity allows detection of HI in all but a few isolated regions of the
areas between the spiral arms. Most of the atomic hydrogen is located within
the optical disc, but the HI extends to some 1.5R_opt. The HI follows the
spiral arms, and streaming motions of up to ~15 kms (projected) can be
identified from the velocity field. Assuming a constant inclination angle of 17
deg, a rotation curve is derived which is slightly falling in the outer parts
of the disc. Analysis of a residual velocity field, obtained after subtraction
of an axisymmetric model based on the rotation curve, confirms the existence of
streaming motions near the spiral arms in an otherwise undisturbed disc.
