We have investigated the effect of lowering foetal arterial PO(2) either acutely or chronically on tyrosine hydroxylase (TH) protein content in the dorsal and ventral medullary regions of the brainstem of the sheep foetus during late gestation. TH protein content increased in both the dorsal and ventral medullary regions of the foetal brainstem after exposure to acute hypoxia when compared to normoxia. In contrast there was no increase in the TH protein content of either the dorsal or ventral medullary regions in the brainstem of foetal sheep which were chronically hypoxaemic throughout late gestation as a consequence of experimental restriction of placental growth. The differences between the TH responses to acute and chronic hypoxaemia in the foetal sheep brainstem may be important in the mediation of physiological adaptations to these intrauterine stimuli and for the generation of an appropriate physiological response to hypoxia in the newborn period.