5,7-DHT (104 micrograms) was injected into the third brain ventricle of three-day-old rat pups 1 hr after desmethylimipramine pretreatment (20 mg/kg IP). Whole-brain serotonin was depleted by 50%, and there was some retardation of weight gain. Nipple attachment was observed in sucklings and in weaned pups. When 9 days old, attachment in 5-HT depleted pups was delayed and they were less active when undeprived, but not when 8 hr deprived. In comparison with pups raised in litters of 6 or 14, which served to control for reduced body weight, depleted pups' behavior was more similar to the heavier pups, suggesting that the behavioral differences were not due to retarded growth. After weaning at 21 days, depleted pups attached more up to 37 days of age, after which attachment was not reinstated by treatment with methysergide, suggesting that the waning of depletion induced attachment was not due to recovery of serotonergic function. Again, body weight controls showed that the effects were not due to retarded weight gain in the depleted pups. The results confirm the biphasic action of 5-HT antagonism on attachment, i.e., inhibition in the neonate and disinhibition in the weanling. The findings also show that central 5-HT systems are involved in the inhibition of attachment during weaning, but that this involvement is transient, and apparently not crucial.