TT-235 is a potent oxytocin (OT) antagonist that blocks the action of OT at the receptor level. Previous studies have shown that pregnant baboons demonstrate nocturnal uterine contractions induced by OT as they near delivery. The purpose of this study was to evaluate the changes in plasma OT levels following uterine contraction blockage with TT-235. A tethered pregnant baboon model in its last trimester of pregnancy was used. Three blocks of arterial blood samples, immediately before, plus 1 h and plus 2 h following an OT antagonist injection, were collected once nocturnal uterine contractions were detected. Each block consisted of a continuous 10 min withdrawal with 10 samples per block (1 ml/min). A TT-235 dosage of 300 micrograms/kg and saline for control were utilized. Uterine activities were monitored as pressure changes in the amniotic fluid, and the frequency and mean amplitude of contractile activity per 10 min intervals were expressed as contractile force. Plasma OCT levels were determined by a radioimmunoassay following plasma extraction with petroleum ether. The contractile force was decreased by 77% (p < 0.05) within 2 h after TT-235 administration while it increased 23% following saline infusion. Plasma OT levels were unchanged following saline infusion while they increased 82% (p < 0.05) 2 h after the administration of TT-235. If a positive feedback existed between uterine contractions and OT release, one would expect plasma OT levels to be decreased with contractile activity following TT-235 infusion. Since this is not the case in the present study, the data suggest that there is either a negative feedback or an independent relationship between nocturnal uterine contractions and OT release.