Expressions for the distributions of the density and mass of spiral galaxies have been derived for three different orientations of rotation curves with a turn over radius (r t). The central region of the galaxy is found to rotate as a rigid body with a constant density and with a central mass as a function of r 3. The density of the outer regions (r > r t) is found to vary inversely with the radius (r), and the corresponding mass is found to be a function of r 2. Galaxies with flat rotation curves at (r > r t) has less mass than that with rising up or lowering down rotation curves.