Previous studies have shown that prolactin administration to pregnant rats results in offspring with enhanced abundance of the brown adipose tissue-specific uncoupling protein (UCP) 1. The present study therefore aimed to determine whether a similar effect was observed after birth and if the sex of the animal further influenced the responsiveness of brown adipose tissue mitochondria to prolactin administration. Daily prolactin injections were therefore commenced at 15, 35 or 60 days of age and continued for 4 days. The abundance of UCP1 was unchanged with age in females but decreased between 15 and 35 days in males and was lower in males than females by 60 days of age. Cytochrome c abundance remained unchanged with postnatal age in both males and females and was consistently higher in males at each sampling age. Prolactin decreased the abundance of UCP1 and cytochrome c when administered to female rats at 35 and 60 days of age, but had no effect at 15 days. In contrast, prolactin had no effect on UCP1 in male rats at any age, but did stimulate the abundance of cytochrome c at 15 days of age. In conclusion, the administration of prolactin to postnatal rats over the period in which maturation of the hypothalamic-pituitary axis and brown adipose tissue function is occurring did not enhance UCP1 abundance. In females, prolactin actually caused a reduction in UCP1 suggesting that in rats it is only prior to birth that prolactin has a stimulatory role on brown adipose tissue development. Experimental Physiology (2003) 88.4, 527-531.