The effect of reserpine and 6-hydroxydopamine on the learned conditioned natural killer (NK) cell response was investigated in mice. Reserpine given at 2.5 mg/kg, 24 hr prior to reexposure to camphor-conditioned stimulus on days 6 and 8 blocked the recall of conditioned NK cell response to a significant extent. In other words, the NK cell activity of conditioned mice, treated with reserpine and reexposed to the conditioned stimulus, was similar to the nonconditioned (NC) group. A conditioned increase in NK cell response was still evident in mice treated with 6-OHDA. © 1990.