Data from neutral hydrogen maps of "A 31 taken at 53 velocities have been used to investigate the velocity field over the surface of the galaxy. The rotation curve which is determined in two independent ways shows in each case significant differences between the NF and SP major axes as suggested by previous investigations. Both the velocity and hydrogen distribution information can be used to derive the fundamental parameters of M 31. We find the tilt of the plane of the galaxy to the line of sight is I 201 1 0, the position angle of the major axis is 3801 1 0 and system velocities in the range -299 to -311 km -1 relative to the local standard of rest, depending on the method of estimation. A small velocity asymmetry was found in the quadrants of M 31 which is in the sense to be expected if the NW and SE quadrants were tilted towards the line of sight (a smaller tilt angle). Some individual regions also have peculiar velocities. An investigation of a possible expansion of the neutral gas indicates that the expansion is less than 10 km -1 at 10 kpc from the centre. The agreement between the velocity of groups of H II regions and neutral hydrogen at the same position is very close (within 1 10 km s-1). A mass distribution derived from the rotation curves follows the blue luminosity distribution 20' and 8 ' arc from the centre with AlIL = 12+2. The total mass of the system out to 150' arc from the centre is estimated as X 1011 solar masses and the Al/L ratio is .