1. Isolated segments of bovine mesenteric lymphatic vessels were loaded with [3H]-noradrenaline and its efflux in response to field stimulation at 2 Hz (0.3 ms pulses, 1 min train) examined. 2. Isoprenaline (10(-6) M) increased evoked fractional 3H efflux (3H released as a percentage of total tissue 3H at the onset of stimulation) from its control value of 0.67 +/- 0.09 to 1.2 +/- 0.18% (s.e.mean; n = 5). 3. Propranolol (10(-6) M) alone had no effect on evoked fractional 3H efflux but blocked the increase in transmitter overflow induced by isoprenaline (10(-6) M). 4. In the presence of the alpha-adrenoceptor antagonist phentolamine (10(-6) M) the facilitatory effect of isoprenaline (10(-6) M) on transmitter efflux was enhanced. In 5 experiments isoprenaline increased evoked fractional 3H efflux from its control value of 1.07 +/- 0.17 to 2.5 +/- 0.37% when phentolamine was present. 5. Adrenaline (10(-8) M) increased evoked fractional 3H efflux from its control value of 0.76 +/- 0.09 to 1.13 +/- 0.17% (n = 7). 6. These findings are consistent with the presence of presynaptic beta-adrenoceptors in this preparation. There is no evidence that these receptors play any role in feedback regulation of transmitter release.