Janik, J., Callahan, P., and Rabii, J. (1993). Morphine induced analgesia is attenuated in post-partum lactating rats. LIFE SCIENCES 52 (3): 271-279. The analgesic effects of morphine administration were determined in post partum, lactating female rats, as well as in intact, cycling females during the diestrous stage of the estrous cycle. All doses of morphine (2.5, 5 and 10 mg/kg, iv) produced a significant analgesic response in both post-partum and diestrous females using the hot water tail immersion latency test. However, the analgesic response in the post-partum females was significantly less than during diestrus at all doses tested. In addition, pretreatment with the mu 1 specific antagonist, Naloxonazine, significantly blunted the analgesic response in diestrous females, but did not significantly affect analgesia in post-partum females. These results indicate that morphine is less effective in producing analgesia in post partum females. The mu 1 opiate receptor site does not appear to be involved in the analgesia produced during the post-partum period.