For time- and velocity-dependent neutron transport theory, a reciprocity theorem is discussed that expresses a symmetry property of point-source or plane-source solutions in systems of uniform temperature. As a consequence of the theorem, the coefficients in a decay-mode expansion are found to be the adjoints of the normalized modes. A long-time solution of the time-dependent albedo problem for a thick slab is derived from diffusion theory, with the aid of the reciprocity theorem. The reflected and transmitted net currents and angular densities are expressed in terms of the solution of the steady-state Milne problem.