The effects of differential mating stimulation on fertility in rats were examined by mating pro-oestrous females for one ejaculatory series in tests in which they could or could not self-regulate, or pace, the timing of intromissions received by males. Females were autopsied on days 7, 14 or 21 after mating, or on the expected day of birth to confirm pregnancy, and the number of implantation sites or of viable fetuses or pups determined. Because of substantial behavioural variability within an ejaculatory series, data from paced and nonpaced females were divided according to whether they received a low (< or = 8) or high (> or = 9) number of intromissions. The incidence of pregnancy was significantly reduced among paced females receiving few intromissions relative to that of any other group. Histological examination of ovaries from females autopsied on day 7 after mating suggested that the reduced pregnancy rate among this paced, low intromission group resulted from a failure of activation of the corpora lutea, a possible consequence of the low number of intromissions received by these females. However, in paced, low intromission females that became pregnant, litter size was significantly greater than in nonpaced, low intromission females. These results suggest a compensatory effect of the temporal patterning of intromissive stimulation on fertility. This effect is not a consequence of differential mortality after implantation since there was no difference in litter size among females autopsied at any of the four times. The differences between paced and nonpaced females may be attributable to preimplantation effects such as differential release of ova or sperm transport.