An exposure for 60 min to a 0.5 Hz rotating magnetic field (1.5-90 G) significantly reduced the day-time analgesic (in CF-1 mice) and locomotory (in C-57BL mice) effects of morphine (10 mg/kg). Intracerebroventricular (i.c.v.) injections of a calcium chelator, EGTA, blocked these effects, while administration of the calcium ionophore, A23187, potentiated the inhibitory actions. In a parallel fashion, i.c.v. administration of Ca2+ reduced, in a dose-related manner, the analgesic and locomotory effects of morphine in control CF-1 and C57 mice. These latter inhibitory effects could also be blocked by EGTA and augmented by A23187, indicating that opiate effects on activity and nociception are both sensitive to antagonism by calcium. Taken together these results suggest that exposure to magnetic stimuli may alter morphine-induced responses in mice, in a manner compatible and consistent with effects on Ca2+ and possibly other divalent ions.