To examine the effects of 0, 50 and 100 rad of 60Co administered prenatally on postnatal development of neuromuscular coordination, visual discrimination learning, spontaneous light-dark stabilimeter activity, plasma cortisol and somatometric growth rates, diurnal squirrel monkeys were studied from birth to 90 days. In terms of accuracy, completeness and time required for performance of reflexes and neuromuscular coordination, the performance of 50- and 100-rad offspring was less accurate and poorly coordinated and required more time for completion compared to that of controls. In visual orientation, discrimination and reversal learning, the percentage correct responses of the 50- and 100-rad offspring were significantly lower than those of controls. Spontaneous light-dark stabilimeter activity of 50- and 100-rad offspring was significantly higher in the dark session than that of controls. Plasma cortisol was significantly higher in 100-rad infants than in controls. Comparisons of somatometric growth rates indicated that postnatal head circumference, crown-rump length and, to a lesser extent, body weight increased at significantly slower rates in 50- and 100-rad offspring. These findings should provide essential information for formulating and carrying out multivariate behavioral, biochemical and morphometric assessments of low-dose effects on the brain of primate offspring within demonstrable dose-response curves.