This study examined the feasibility of using colorectal distention as a noxious visceral stimulus in rat pregnancy-induced analgesia studies as well as the influence of naloxone on the pregnancy-induced changes in the behavioral response to a noxious visceral stimulus. In the first part of the study, we compared the effects of pregnancy on several forms of noxious stimulation (colorectal distention, hypertonic saline induced writhing, tail flick, and hot plate). After determination of prepregnant baseline values, one group of rats (n = 35) was mated and retested on days 7 and 21 of gestation and 1, 3, 5, 7, and 14 days after parturition. After baseline determinations on day 21 and postpartum day 1, the animals received a subcutaneous injection of naloxone 1.0 mg/kg and were retested. A nonpregnant control group of animals (n = 7) was tested in the same manner. On day 21 of gestation and postpartum days 1 and 3, significant changes (higher threshold or longer latency to response) were observed after all but the writhing test. High-dose naloxone (1.0 mg/kg) significantly reduced the increases observed on day 21 and post-partum day 1. Nonpregnant rats demonstrated no significant change on any test day. The second part of the study evaluated a possible influence of low-dose naloxone on pregnancy-induced analgesia to visceral stimulation (colorectal distention) in another group of pregnant rats (n = 44). The significant increase in threshold on day 21 of gestation was not changed by intravenous (i.v.) naloxone 1.0, 2.3, or 3.0 micrograms/kg, whereas 5.0 and 20.0 micrograms/kg naloxone significantly decreased the observed pregnancy-induced analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)