The aim of this study was to test the hypothesis that, in non-human primates, the offspring-mother attachment and other social bonds within the group have in common neural mediating mechanisms involving brain opioids. The subjects were 10 juvenile macaques living in a stable social group with their mothers and other group companions. A within-subjects design, balanced for the order of drug administration (naloxone 1 mg/kg i.m. and saline), was used. In the naloxone condition, the juveniles increased their relative role in maintaining proximity with their mothers, made more grooming solicitations, and received more grooming. We found no evidence for differential effects of naloxone on the affiliative bonds the subjects had with their mothers and other group companions. The subjects' increased demands for social comfort were evenly addressed to, and responded to by, both their mothers and other group companions. This finding supports the hypothesis that, at a neural level, the endogenous opioid peptides form a common substrate for different types of social attachments in primates.