Right adrenal medullary and various cardiovascular responses to stimulation of the peripheral end of the right splanchnic nerve have been investigated in conscious calves 24-48 h after birth and compared with those obtained previously in older conscious calves. The results show that splanchnic nerve stimulation produces a smaller rise in mean aortic blood pressure and haematocrit in new-born calves than occurs in older animals. The fall in adrenal vascular resistance is also reduced, as is the amount of catecholamine released from the adrenal medulla, at 24-48 h. Although the absolute amounts of adrenaline and noradrenaline released in response to splanchnic nerve stimulation were significantly less in 24-48 h old calves, the ratio (3:2) was the same as that found in 2-5 week old animals and that in which the two amines are stored in the gland. The results confirm the contention that the sympathetic innervation is relatively immature immediately after birth in this species and that functional maturation occurs gradually over a period of weeks. However, they also show that the ratio in which adrenaline and noradrenaline are released from the adrenal medullae provide no index of this process, as had previously been suggested.