Bovine mesenteric lymphatics of 8 cm length were cannulated and arranged so that pressure changes produced by spontaneous contractions and accompanying flow could be measured. The mean frequency of spontaneous contractions, although constant for any one vessel, varied between vessels from 0.5 to 5.2 beats/min (mean 2.6, S.D. 1.6, n = 33). Noradrenaline (100 ng/ml.) invariably increased frequency of contractions but flow was increased only in those vessels which had a low resting frequency. In those vessels which had a higher resting frequency, flow was decreased despite the increase in frequency because stroke volume decreased. This may have been due to increased pace-maker activity causing desynchronization of the contractile activity in the vessel wall and thus decreased effectiveness of pumping. High doses (greater than 1 microgram/ml.) of noradrenaline invariably depressed flow. Isoprenaline slowed the frequency of spontaneous contractions and this either had little effect on flow (due to an increased filling time and thus stroke volume) or depressed it, finally abolishing it entirely at higher doses. It is concluded that in suitable dosage (which varies for different vessels) noradrenaline is capable of enhancing flow and this might, at least in part, explain the increase in lymph flow resulting from noradrenaline infusion in living animals.