In control intact rats (group C), the frequency of intrauterine pressure cycles (IUPC) declines steadily during pregnancy from 80-130 cycles/3 h on day 10 (D10) to 20-30 cycles/3 h on D20. The decline in frequency is due to increasingly prolonged periods of myometrial quiescence, which increase from 30-90 min/3 h on D10 to 120-150 min/3 h by D20. During the 24 h preceding labor, the frequency of IUPC remains at less than 15 cycles/h until 3 h prepartum, when there is an abrupt increase to 30 cycles/h. Ovariectomized pregnant animals treated with progesterone (P) and estrogen (E; group OPE) exhibit significantly greater frequency of IUPC and significantly lower incidence of myometrial quiescence than intact pregnant rats from D12 and for the remainder of pregnancy. Ovariectomized pregnant rats that received P, E, and porcine relaxin (R: group OPER) or P and porcine R only (group OPR) exhibited declining frequency of IUPC similar to that observed in group C animals. Group OPER rats exhibited prolonged periods of myometrial quiescence of similar duration to those observed in control intact rats. In group OPR animals, however, the periods of myometrial quiescence were considerably diminished during late pregnancy. Group OPER animals exhibited a pattern of myometrial activity during labor and postpartum similar to that of control animals. The results suggest an important role for R in the control of myometrial activity during the second half of pregnancy and parturition in the rat.