Using [3H]DA as the ligand of choice, we have shown that rat thymocytes possess dopamine binding sites. The binding of [3H]DA was specific, saturable and of high affinity. [3H]DA binding was rapid and reversible; [3H]DA was totally displaced by low concentrations of dopamine, norepinephrine, and apomorphine. The binding sites were insensitive to neuroleptic drugs such as sulpiride, bromocriptine, and haloperidol. On the basis of these findings, we have suggested that thymocytes possess a D-3 receptor type.