1 Stimulation of periarterial nerves to the ileum of 1 to 12 day old rabbits with supramaximal voltages and frequencies of 1, 2, 5, 10 and 20 Hz with square wave pulses of 2--5 ms duration for 30--40 s produced responses that were initially contractor. In the course of the first week, the responses changed from motor to inhibitory, the change occurring first at the highest rates of stimulation. By the 7th day of life, almost all responses were inhibitory. 2 The motor responses were potentiated by physostigmine and blocked by hyoscine suggesting that they were mediated by acetylcholine. 3 In preparations from rabbits older than 3 days, motor responses could be converted to inhibitory ones by prior exposure to noradrenaline (NA, 1 microgram/ml) for 20 min. This procedure also significantly increased the responses which were already inhibitory. 4 The ability of the ileum to take up NA increased with age. This uptake was blocked by cocaine. 5 The following explanations are possible: (a) changeover from cholinergic to adrenergic transmission in sympathetic fibres; (b) existence of 'parasympathetic' splanchnic nerves or a permanent cholinergic 'sympathetic' component of splanchnic nerves and (c) temporal delay in the development of adrenergic nerves (compared with cholinergic nerves) in the intestine.