1. Different patterns of stimulation have been applied to the peripheral end of the right splanchnic nerve, below behavioural threshold, in conscious 2-5 week-old calves. 2. The effects of continuous stimulation at 4 Hz for 10 min were compared with those of stimulation at 40 Hz in 1 s bursts at 10 s intervals for the same period. Delivering the same total number of impulses in the form of bursts in this way increased the output of both adrenaline and noradrenaline and this increase was statistically significant in the case of adrenaline (P less than 0.02) but not noradrenaline. 3. The effects of stimulation in 1 s bursts at 10 s intervals for 2-3 min were investigated over the frequency range 10-150 Hz and compared with the effects of continuous stimulation over the range 1-15 Hz obtained previously in conscious calves of the same age (Edwards, Furness and Halle, 1980). The output of adrenaline but not noradrenaline was found to significantly greater in response to stimulation in bursts at frequencies of up to 40 Hz than when the equivalent number of impulses were delivered at a constant rate (P less than 0.02). 4. It is concluded that the release of catecholamines from the adrenal gland is maximal at relatively high frequencies (40-100 Hz) when the impulses are delivered in bursts.