A critical evaluation of the literature and a cost/benefit analysis suggest that changes in affiliation patterns in female monkeys following the birth of infants should be mediated by female reproductive state and dominance rank and infant characteristics such as age and sex. Several predictions concerning the effects of these variables were tested on dyadic interactions between lactating pigtail macaque (Macaca nemestrina) females and other lactating or pregnant females. Dyads composed of lactating females spent more time in proximity, contact and grooming than dyads composed of lactating and pregnant females. Association with other lactating females was mainly due to lower-ranking females that may gain benefits such as increased tolerance and support from their partners. Infant handling by lactating females had a higher percentage of harassment episodes than handling by pregnant females. Among dyads composed of two lactating females, affiliative interactions were inversely correlated with rank distance. It is suggested that the presence of infants may influence female social dynamics at the group level to a greater extent than previously thought.