The used of selective serotonin reuptake inhibitors (SSRIs) by depressed parturients has raised concern over possible teratogenic effects on the fetus. These drugs readily cross the placental barrier, exposing the fetus to increased serotonin levels early in development. This prospective study evaluated infant neurobehavior 14 to 39 hours after birth; all were normal-birth infants. Half the mothers were taking an SSRI. The users and nonusers were matched for cigarette smoking, maternal age, and socioeconomic status. Infants were monitored for behavior, startles, and tremulousness for 1 hour between feedings. Motor activity and heart rate variability (HRV) were recorded during a 15-minute sleep period. Behavioral assessment was based on the Neonatal Behavioral Assessment Scale. The SSRI-exposed infants had shorter gestational ages. They exhibited more motor activity and tremulousness than did unexposed infants and had fewer rhythms in HRV. They also had fewer changes in behavioral state and fewer such states. Peak behavioral state was lower in the exposed infants. These infants also had significantly more rapid-eye-movement sleep and tended to have more spontaneous startles and sudden arousals. After adjusting for gestational age, the only significant differences were in tremulousness and measures of state and sleep organization. Adjusted mean motor activity and spectral peaks in HRV no longer differed significantly between the SSRI-exposed and nonexposed infants. These findings do not support the view that using an SSRI during pregnancy has little effect on the developing fetus. It is possible that the observed changes in motor activity, startles, and HRV are mediated by the effects of SSRI exposure on gestational age.