The effects of depleting the tissue catecholamines with alpha-methyl-p-tyrosine (AMPT, an inhibitor of catecholamine synthesis) on the splenic NK cell cytotoxic activity were assessed both in vivo and in vitro. Treatment with i.p. doses of AMPT (3-500 mg/kg) produced a dose-related suppression in the splenic NK cytotoxic activity in many mouse strains. For an i.p. dose of 300 mg/kg of AMPT, the splenic NK cell activity began to fall 1 h after AMPT injection. The splenic NK cell activity reached its maximal level at 3 h, accompanied by a severe depletion of norepinephrine contents in the spleen. Both the NK activity and the splenic norepinephrine content returned to their control levels at 24 h. Direct addition of AMPT (up to 2500 micrograms/ml) to the cultured mouse spleen cells in vitro resulted in no NK suppression. However, when the serum obtained from the AMPT-treated mice was added to the cultured mouse spleen cells of the AMPT-untreated mice, the splenic NK cytotoxic activity was greatly suppressed. In addition, natural killing by spleen cells from AMPT-untreated mice was not reduced by the addition of spleen cells from AMPT-treated mice. No evidence of AMPT-induced cellular suppressors of natural killing could be detected. It was also found that both the splenic NK cell activity and the effector-target cell conjugation activity were suppressed by treatment with AMPT. These observations indicate that depleting the tissue catecholamines with AMPT results in the release of certain humoral factors which can suppress both the effector-target cell conjugation activity and the splenic NK cell activity in mice.