Oxytocin (OT)-induced uterine contractility in different ovulatory phases has not been characterized in mouse. While the uterus was refractory to OT at diestrous phase, numerous contractions were observed at estrous phase. The magnitude of uterine contractile response to OT at estrous phase was larger than that at diestrous phase. There was also significant difference in the pD(2) values for OT between the samples from the two phases. However, the level of OT receptor (OTR) transcripts and proteins was equivalent at the two phases. These results suggest that OT-induced uterine contraction is not only regulated by the quantity of OTR in the non-pregnant mouse uterus. (C) 2004 Elsevier Inc. All rights reserved.