Women's reproductive biology imposes heavy obligatory costs of parental investment, creating strong selective forces hypothesized to shape female mating psychology around critical decisions such as the choice of partner, the timing of sexual intercourse, and the timing of reproduction. We propose that female sexual desire has evolved as one adaptation among several designed to regulate these decisions. We hypothesize (a) an increase in desire as conception probability increases, but only among women who are in committed long-term relationships; and (b) a shift in the desire for a primary partner as compared with extra-pair partners as ovulation approaches, dependent upon a woman's evaluation of her primary partner's relative quality. We tested several predictions derived from these hypotheses in a study of 173 women who were not taking oral contraceptives. Results confirmed Hypothesis 1: An ovulatory peak in sexual desire was found only for mated women; for unmated women, conception probability and sexual desire were uncorrelated. Hypothesis 2 was partially supported. Among mated women, those with higher conception probability exhibited higher levels of in-pair sexual desire relative to those at lower conception probability. Conception probability and relationship length interacted significantly to predict extra-pair desires, such that women in longer relationships were more likely to experience desire for extra-pair partners during periods of high conception probability. The pursuit of an in-pair conceptive strategy (as opposed to an extra-pair conceptive strategy) was also associated with the occurrence of sexual activity in the relationship.