Although the sexual behavior of female rhesus macaques (Macaca mulatta) has been reported to decrease during pregnancy, systematic studies have been largely confined to reports of male behavior. Therefore, we designed a study to examine changes in female sexual behavior during pregnancy. Six cycling multiparous female rhesus macaques were paired during the periovulatory period of the menstrual cycle and during the first and second halves of pregnancy with six sexually experienced males of the same species. Each female was paired with the same male in all tests. The means of the behavioral measures scored on days 11-13 of the menstrual cycle, 35-56 of pregnancy, and 147-168 of pregnancy were compared. Females showed a significant decrease in the present contact ratio and the rate of threat-aways during pregnancy. Males showed lower rates of threatening-away, contacting, mounting, intromitting, and ejaculating when females were pregnant. The extent to which the decrease in male sexual behavior can be accounted for by an increase in female rejection is uncertain, but it is probably as significant as any loss of attractiveness that might accompany pregnancy.