The usual kinetic description of a plasma is extended to include variables to describe the nuclear spin. The distribution function, over phase space and the new spin variables, provides a sufficient description of a spin‐polarized plasma. The evolution for the distribution function is given. The equations derived are used to calculate depolarization caused by four processes: inhomogeneous fields, collisions, collisions in inhomogeneous fields, and waves. It is found that depolarization by field inhomogeneity on scales large compared with the gyroradius is totally negligible. The same is true for collisional depolarization. Collisions in inhomogeneous fields yield a depolarization rate of order 10−3sec−1for deuterons and a negligible rate for tritons in a typical fusion reactor design. This is still sufficiently small on reactor time scales. However, small‐amplitude magnetic fluctuations (of order 1 G) resonant with the spin precession frequency can lead to significant depolarization (depolarizes triton in 10 sec and deuteron in 100 sec).