Isolated vas deferens preparations from 16 rabbits of the New Zealand white strain were subjected to electrical and chemical excitability under physiological conditions and under the influence of drugs. Such smooth muscle fibres were strictly confined to the terminal 3 cm. segments of the distal 'urethral' portions of the vasa deferentia. Intermittent field stimulation, at 60 second intervals, was provided by a stimulator of low output impedance under constant parameters of voltage, pulse width and frequency. Results from this investigation revealed the undermentioned anomalous but distinct findings viz: (a) the presence of a minor adrenergic component which disappeared in phentolamine but remained unaffected by prolonged exposures to phenoxybenzamine; (b) the presence of a predominantly non-adrenergic component which was totally refractory to phenoxybenzamine but suffered a weak diminution in phentolamine; and (c) the picture of a phentolamine-insensitive but twitch-inhibiting effect shared by both tyramine and noradrenaline. Rabbit vasa consistently displayed a remarkable insensitivity to the motor effects of submicromolar concentrations of the putative neurotransmitter substance i.e. noradrenaline. The indirect sympathomimetic agent, tyramine failed to elicit any contractions when employed in doses as massive as 290 micrometers. i.e. 5 x 10(-5) g/ml, except in 2 out of 14 trials when a weak phentolamine-susceptible bursts of single contractions were produced. Thus, the adrenoceptors involved in the mediation of the twitch-inhibiting effects, appear not to behave towards conventional adrenoceptor antagonists in the classified manner. It seems appropriate therefore to invoke an unknown neurotransmitter for the non-adrenergic component in this motor transmission.