Behavioral and electroencephalographic (EEG) effects on a new macrolide antibiotic miokamycin (MOM), 9,3"-diacetylmidecamycin, were investigated in mice, rats and rabbits after oral administration. MOM caused a decrease in the spontaneous motor activity, a slight increase in the effect of megibal seizure at a dose of 1,000 mg (Potency)/kg, and an increase in the number of deaths on the maximal electroshock seizure after administration of doses more than 100 mg (Potency)/kg. Even at a dose of 1,000 mg (Potency)/kg, however, MOM showed no effects on general behavior, rectal temperature, traction test, inclined screen test, rotarod performance, thiopental-induced sleep, pentetrazol seizure, fighting behavior induced by electric stimulation and conditioned avoidance response. EEG effects in unanesthetized rabbits with permanent electrode implants were studied. MOM showed no effects on spontaneous EEG, arousal response to mesencephalic reticular stimulation, photic driving response, evoked cortical response by ventralis posterolateralis stimulation and afterdischarge elicited by hippocampal stimulation at a dose of 1,000 mg (Potency)/kg. Consequently, it can be concluded that MOM has no specific pharmacological effects on the central nervous system. Behavior effects of MOM metabolisms, Mb-1, Mb-2, Mb-6 an Mb-12, were almost the same as those of MOM in the mode of action.