An adenylate cyclase activity (AC) was found in guinea pig brown adipose tissue (BAT), since the tissue's apparition. This enzymatic activity increased during the development and showed high values at the end of gestation. An increase of AC units per cell was observed, in addition to the cell multiplication. A norepinephrine stimulation of AC activity was observed at the end of gestation : this regulating action disappeared in the first days of extrauterine life. Neither glucagon nor ACTH had any regulating role upon AC activity during fetal and newborn life. The basal lipolytic activity which was observed in BAT of fetuses (61rst day) and neonate dramatically around the 15th day. A potent lipolysis activation by norepinephrine was observed, but only after birth. The correlation observed between these enzymatic activities in presence of norepinephrine seems to indicate that the AC/lipase system was involved in the neonatal thermogenesis of guinea pigs.