We are investigating the effect of prenatal exposure to endocrine disrupting chemicals (EDCs) on the social development of two species of macaques (mulatta and fascicularis). There is no reliable body of research or evidence that EDCs such as dioxin, bisphenol A (BPA) and polychlorinated biphenyls (PCBs) affect the central nervous system (CNS). We evaluated the relationships between these chemicals and behavioral characteristics in both mother-infant or peer interactions in order to clarify the effect of EDCs on the CNS. All primiparous monkeys were singly housed in stainless-steel cages based on the NIH guidelines (69 cm x 61 cm x 75 cm) during their pregnancy and offspring-care period. Animals were supplied with daily solid diet and water ad libitum. The offspring was separated from the mother after weaning and singly housed from 6 until 12 months of age. We found smaller effects of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (2, 3, 7, 8-TCDD) than expected, but serious effects of BPA and PCBs. BPA prevents behavioral sex differentiation in infants, and PCBs suppress their social initiative in mother-infant and peer interactions. These studies proved that the observation paradigm method was available and helpful for assessing the subtle influence of EDCs in the brains of higher primates.