Attention and learning problems among children exposed to opiates in utero have been previously reported but are difficult to interpret due to imprecise measurement and inadequate control of postnatal factors. In this study, we used a behavior-based measure of attention (continuous-performance tasks) and a physiological index of sustained attention (cardiac vagal tone) to measure more precisely the process of sustained attention. Boys, aged 7 to 12, exposed to opiates in utero, were compared to boys whose mothers began using illicit substances after the child's birth (environmental controls), and boys whose mothers were non-drug users. This three-group design was intended to isolate in utero effects from postnatal environmental influences. Vagal tone, a measure of heart-rate variability sensitive to vagal influences on the heart, was measured pre- and postbaseline and during the three tasks of the Gordon Diagnostic System (Delay, Vigilance, and Distractibility). Vagal tone has been found to be sensitive to changes in environmental demand for sustained attention in infants, school-age children, and adults. Results indicated that when distractors were added to the vigilance task (Distractibility task), opiate-exposed boys failed to suppress vagal tone compared to both control groups. However, both the opiate-exposed boys and the environmental controls made fewer correct responses than non-drug-exposed controls on this task. These results indicate that normal physiological responses to increased attentional demand may be impaired in boys exposed in utero to opiates in this age range. However, the poor Distractibility scores of both the opiate-exposed and environmental controls suggests an important role of environmental influences on attentional performance.