1. Electrical field stimulation (10 Hz, 60 V, 1 ms, for 10 s) produced monophasic contractions of isolated preparations of rat cauda epididymis which could be abolished by guanethidine, and attenuated by prazosin and alpha, beta-methylene ATP. 2. The rank order of potency of adrenoceptor agonists in causing contraction of the preparation in the presence of the neuronal uptake blocker, nisoxetine (0.1 microM) was: adrenaline greater than or equal to phenylephrine greater than or equal to noradrenaline greater than clonidine greater than methoxamine greater than metaraminol greater than dopamine greater than or equal to isoprenaline greater than xylazine. 3. Responses to the agonists were blocked by prazosin but not by propranolol or idazoxan. 4. The rank order of potency of purinoceptor agonists in causing contraction of the cauda epididymis was: alpha,beta-methylene ATP greater than beta,gamma-methylene ATP greater than or equal to 2-methylthio ATP greater than ATP greater than ADP. AMP and adenosine did not cause contractions. 5. Contractile responses to the purine nucleotide analogues were blocked by repeated application of alpha,beta-methylene ATP. 6. It is concluded that both ATP and noradrenaline may act as co-transmitters in the sympathetic nerves supplying the smooth muscle of the rat cauda epididymis, and that alpha 1-adrenoceptors and P2x-purinoceptors are present postjunctionally.