The behavior of eight adult male marmosets that were bilaterally castrated during neonatal life (postnatal days 1–7), a single unilaterally castrated male, and eight age-matched intact adult males was observed during encounter tests with unfamiliar conspecifics. When neonatally castrated (NC) males encountered ovariectomized, estradiol-treated females, both partners were intensely aggressive and 66% of tests were terminated to avoid injuries. No mounts occurred, and females exhibited little or no sexual initiating behavior (proceptivity) towards NC males. By contrast, encounters between NC males and intact adult males were characterized by low levels of aggression and no tests were terminated. Intact males frequently mounted NC males. However, NC males initiated only 4% of these mounts by intact males and they refused or terminated 67% of mount attempts and mounts by intact males. The age-matched intact adult males provided data which differed markedly from those obtained using NC males. The intact males were highly aggressive during encounters with other intact males, whereas they mounted and ejaculated when paired with females. The single male which had been unilaterally castrated in infancy provided similar results to the intact males. These experiments indicate that neonatal castration has profound effects upon the development of sexual and aggressive behavior in male marmosets. However, the stimulus qualities as well as the behavior of males are altered by neonatal castration, so that unfamiliar conspecifics react towards them as females during paired encounters. This finding complicates the interpretation of effects of neonatal castration upon development of sexually dimorphic behavioral patterns in male marmosets. Behavioral effects of castration later in postnatal development romain to be determined. © 1993 Wiley-Liss, Inc.