Atomic hydrogen in the spiral galaxy NGC 3631

New high resolution, high sensitivity WSRT HI synthesis observations of thespiral galaxy NGC 3631 are presented. In the total atomic hydrogen map, thespiral arms are well distinguished from the interarm regions, while thesensitivity allows detection of HI in all but a few isolated regions of theareas between the spiral arms. Most of the atomic hydrogen is located withinthe optical disc, but the HI extends to some 1.5R_opt. The HI follows thespiral arms, and streaming motions of up to ~15 kms (projected) can beidentified from the velocity field. Assuming a constant inclination angle of 17deg, a rotation curve is derived which is slightly falling in the outer partsof the disc. Analysis of a residual velocity field, obtained after subtractionof an axisymmetric model based on the rotation curve, confirms the existence ofstreaming motions near the spiral arms in an otherwise undisturbed disc.Comment: 12 pages, Latex. 10 postscript files included. Better quality versions of the figures available from ftp://star.herts.ac.uk/pub/Knapen/3631hi . Accepted, MNRA