The effect of caesium on the responses of rabbit vas deferens to transmural stimulation was investigated. The tissue responded to transmural stimulation with a phasic spike contraction followed by a sustained contractile response. The sustained response was inhibited by phentolamine and guanethidine and thus apparently results from noradrenaline release from adrenergic nerves. Addition of 2–5 mM Cs+ greatly potentiated this secondary response without altering the sensitivity of the tissue to added (−)-noradrenaline. This potentiation was not due to Cs+ decreasing the neuronal uptake of noradrenaline, or by Cs+ altering prostaglandin synthesis. Addition of 2 mM Cs+ significantly increased the amount of (±)-[3H]metaraminol released from tissues in response to transmural stimulation (5 Hz). It is suggested that caesium potentiated responses of rabbit vas deferens to transmural stimulation by increasing the amount of transmitter released per nerve impulse, possibly as a result of prolongation of the action potential.