Synthetic glucocorticoids such as dexamethasone (DEX) are commonly used to prevent respiratory distress syndrome in preterm infants, but there is emerging evidence of subsequent neurobehavioral abnormalities (e.g. problems with inattention/hyperactivity). In the present study, we exposed pregnant common marmosets (Callithrix jacchus, primates) to daily repeated DEX (5 mg/kg by mouth) during either early (d 42-48) or late (d 90-96) pregnancy (gestation period of 144 days). Relative to control, and with a longitudinal design, we investigated DEX effects in offspring in terms of physical growth, plasma ACTH and cortisol titers, social and maintenance behaviors, skilled motor reaching, motivation for palatable reward, and learning between infancy and adolescence. Early DEX resulted in reduced sociability in infants and increased motivation for palatable reward in adolescents. Late DEX resulted in a mild transient increase in knee-heel length in infants and enhanced reversal learning of stimulus-reward association in adolescents. There was no effect of either early or late DEX on basal plasma ACTH or cortisol titers. Both treatments resulted in impaired skilled motor reaching in juveniles, which attenuated in early DEX but persisted in late DEX across test sessions. The increased palatable-reward motivation and decreased social motivation observed in early DEX subjects provide experimental support for the clinical reports that prenatal glucocorticoid treatment impairs social development and predisposes to metabolic syndrome. These novel primate findings indicate that fetal glucocorticoid overexposure can lead to abnormal development of motor, affective, and cognitive behaviors. Importantly, the outcome is highly dependent upon the timing of glucocorticoid overexposure.