Sex differences in rats' performance on a stationary hidden-platform task (spatial task) in the Morris water maze and the effects of initial nonstationary hidden platform training (NSP training) were examined. The NSP training was designed to familiarize rats with the general requirements of the water-maze task without providing spatial information. NSP training led to faster acquisition and improved retention of the subsequent spatial task in both males and females. There was a sex difference favoring males on acquisition and retention of the spatial task only in rats that had not received previous NSP training. Moreover, there was an apparent reversed sex difference favoring females on some measures of spatial performance in NSP-trained rats. These results suggest that performance on the water-maze task, including the expression of sex differences, can be altered by previous familiarization with nonspatial aspects of the task.

Accumulating evidence shows that magnetic fields can affect a variety of opioid-mediated behavioral and physiological functions. The idea that endogenous opioids are involved in the mediation of fundamental behavioral responses in invertebrates is also gaining support. Evidence exists for opioid involvement in the mediation of nociceptive and antinociceptive ("analgesic") responses of the land snail, Cepaea nemoralis, and other mollusks, in a manner comparable to that in vertebrates. Exposure to various magnetic stimuli, including weak 60 Hz magnetic fields, has significant inhibitory effects on exogenous opiate-induced analgesia and endogenous opioid-mediated nociceptive responses of Cepaea in a manner analogous to that described for vertebrates. These effects of the magnetic stimuli are evident under both laboratory and natural conditions and include disruptions of the day-night rhythms of opioid-mediated nociception. These similar effects in Cepaea and rodents raise the possibility of a phylogenetic continuity in the effects of magnetic fields on basic opioid-mediated biological responses.