Vocalizations of sexually receptive females in two primate species, the sooty mangabey (Cercocebus torquatus atys) and the pigtail macaque (Macaca nemestrina), were compared with respect to the acoustical features of calls as well as the reproductive and social factors that were associated with calling behavior. Sixty-two bouts of calling were recorded from 18 different pigtail macaque females (mean number of bouts per individual-3.48, SD = 2.17, range 1-7) over a six month period; 19.4% occurred during copulation with males, 25.8% were recorded within 30 seconds after copulation has ceased, while the majority, 54.8%, were not associated with mating. The mangabey group yielded 52 bouts from 18 different females (mean number of bouts per individual = 2.89, SD = 2.47, range 1-10) over a comparable period of time; all 52 mangabey bouts were recorded from females during copulation with males. Calls in both species were highly stereotyped and were not acoustically similar to other vocalizations in the species' repertoire; the acoustical structure of the calls of both species, with most energy distributed at relatively low frequencies, suggests adaptations for propagation over distance. Dominance rank of the caller was associated with significant variation in calling by estrus females of both species. There was a strong relationship between rank and bout length in the pigtail females, with higher-ranking females having shorter bouts; the rate of delivery for higher-ranking females was also significantly more variable than it was for lower-ranking females. For the mangabeys, lower-ranking females had significantly higher rates of delivery than did higher-ranking ones. These findings are consistent with the hypothesis that these features of calling relate to a caller's motivational state and that higher levels of sexual motivation are required by lower-ranking females before they show proceptive behavior or mating. The possibility that female-female competition may have a significant effect on important features of calling should be considered in studies that attempt to evaluate the functional significance of these vocalizations.