Carotid chemoreceptor activity was detected in each of fourteen halothane or pentobarbitone anaesthetized exteriorized fetal lambs of 90-143 days gestational age. Activity was about 5 Hz at a Pa,O2 of 25 mmHg and it increased as Pa,O2 was reduced, either by compressing the umbilical cord or by reducing the (FIO2) oxygen fraction of the gas used to ventilate the ewe. Activity increased briskly when 1-2 ml CO2-saturated saline was injected retrogradely into the lingual artery, but not when saline of pH 7.4 or fetal arterial blood was injected. In two fetuses near-term chemoreceptor activity was recorded continuously whilst the umbilical cord was ligated and ventilation with air started. Activity increased 200-500% as Pa,O2 fell, but then fell to below control as Pa,O2 rose. We suggest that these changes in activity reflect those occurring naturally at birth. No spontaneous chemoreceptor activity could be detected on the day of birth in twelve pentobarbitone anaesthetized lambs delivered vaginally or by Caesarean section at 135-146 days. Single baroreceptor activity could however be recorded in these lambs, and chemoreceptor activity could be elicited by hypercapnia. Spontaneous chemoreceptor activity was detected in six of seven lambs more than 2 days old. In eight conscious lambs the steady-state respiratory response to isocapnic hypoxia was variable on the day of birth. In six of these lambs the response was significantly greater by the third day. We conclude that the arterial chemoreceptors are active and responsive in the fetus, but quiescent in the lamb on the day of birth when Pa,O2 has risen. The hypoxic sensitivity of the chemoreceptors is reset from the fetal to the adult range over the next few days.