Postnatal behavior as measured by activity and elimination in the open field, activity in exercise wheels, and hoarding performance, was followed in mice of the ENA strain that had been exposed to 300 R on the day of birth. Long-lasting effects of neonatal irradiation were evidenced by changes in several characteristics. Irradiated mice weighed only 87% as much as controls at 7 weeks of age. Neonatal irradiation was found to decrease elimination scores, activity in wheels, and hoarding scores, but was not found to have a significant effect on open-field activity. Additional studies examined the relationship between locomotion in the open field and different levels of neonatal irradiation in three genotypes of mice. In general, locomotion was observed to decrease with increasing levels of irradiation in the three genotypes, with all three genotypes showing a marked decrease after receiving 300 R. Comparison of present results with those of other studies has demonstrated that strain differences may influence the type of behavioral response observed after neonatal irradiation.