Serotonin exerts an influence on the prenatal development of rat brain. However, later developmental times may be more applicable to the understanding of the role of serotonin in human developmental disorders. Therefore, the current study was undertaken to gain preliminary information on the postnatal effects of serotonin on rat brain development. As the 5-HT1A receptor has been shown to be involved in much of the developmental functions of serotonin, an agonist for this receptor, 8-hydroxy-DPAT (8-OH-DPAT), was used. Neonatal rat pups at three ages (postnatal days, PNDs) 3-10, 10-17 or 17-24) were injected daily with 1 mg/kg 8-OH-DPAT and evaluated for behavioral consequences. The youngest group showed accelerated incisor eruption and eye-opening, a possible consequence of 5-HT1A receptor interactions with epidermal growth factor (EGF). Behaviorally, the animals were more anxious. Animals treated from PND 10-17, showed no change in craniofacial development but showed greater behavioral maturity in measures of spontaneous alternation and activity in the open field. The oldest animals (PND 17-24) showed no behavioral alterations, suggesting that this time length is beyond the critical period for serotonin's influence in brain development.