Sensitive periods for induction of behavioral impairments by developmental iron deficiency were studied in a nonhuman primate model. Rhesus monkey infants were deprived of iron prenatally (n = 14) via the dam's diet (10 microg Fe/g) or postnatally (birth-4 mo, n = 12) via infant formula (1.5 mg Fe/L). They were compared with controls (n = 12) with adequate dietary iron throughout development in a series of cognitive tests and related assessments from 6 to 12 mo of age, a developmental stage corresponding approximately to 2-4 y of age in humans. Health, growth, and hematological status were not affected. Auditory brainstem response and white matter volumes in the cerebrum were similarly unaffected. Male infants in the prenatally deprived group had reduced spontaneous daytime activity relative to controls, as monitored by actimeter. On cognitive tests, prenatally deprived juveniles had similar level of correct responding, but showed more completed trials, and shorter latencies during early phases of the tests. Juveniles deprived of iron as infants showed a similar pattern of behavioral change, but most differences from controls were not as great. Inadequate iron nutrition during pregnancy was reflected in the juvenile period primarily as attenuated inhibitory response. This finding may be relevant to individual differences in temperament or to behavior disorders in children involving reduced inhibitory control.