Acquisition processes of discrete lever-press (L-type) and shuttle (S-type) avoidance responses as well as effects of psychoactive drugs thereon were investigated in dd strain mice. The mice showed a more rapid acquisition of S-type avoidance than L-type. However, the mean avoidance rates and occurrences of good-performing mice (showing an avoidance rate of higher than 75%) were almost the same in both types when the training was carried out for more than 15 sessions of 1 hr each. The response rate of L-type avoidance was 2.5-3 times as high as that of S-type avoidance. Methamphetamine and cocaine increased the response rate in almost the same grade in both types of avoidance. Chlorpromazine, haloperidol, pilocarpine and physostigmine suppressed both L-type and S-type avoidance responses. However, the L-type showed a higher sensitivity than the S-type to the avoidance-suppressing effect of these drugs. Atropine, scopolamine and morphine suppressed L-type avoidance response, while they facilitated S-type avoidance. The drug-induced changes in the response rate of the S-type were well correlated with those in the ambulatory activity. The changes in the response rate of the L-type were also consistent with those in the ambulatory activity after administration of methamphetamine, cocaine, chlorpromazine, haloperidol, pilocarpine and physostigmine, but inconsistent after atropine, scopolamine and morphine. The present results suggest that L-type and S-type avoidance responses in mice sometimes show a different change after administration of psychoactive drugs.