In an attempt to define the pharmacological characteristics of the postjunctional alpha-adrenoceptors of the rat seminal vesicle, responses to certain phenylethanolamine and imidazoline agonists were investigated, in vitro, under experimental conditions outlined by Furchgott (1972), using alpha 1-selective, non-selective and alpha 2-selective adrenoceptor antagonists. Adrenaline (ADR), noradrenaline (NA) and phenylephrine (PE) produced concentration-dependent contractions. In many experiments the concentration-response (C-R) curves had a distinct "shoulder" at the level of 60-80% of the maximum response (Emax), a situation reminiscent of the rat anococcygeus muscle and the rat basilar artery. The relative potencies of ADR:NA:PE, derived from their EC50 values, were 4.07:1:0.26. In contrast clonidine, oxymetazoline and naphazoline failed to contract the tissue even in concentrations up to 1 X 10(-3) M. In fact the imidazoline derivatives prevented responses to the phenylethanolamines. The antagonist action of clonidine, against phenylephrine, was studied in detail. Prazosin, phentolamine, yohimbine, corynanthine and clonidine all caused a rightward displacement of the C-R curves for NA without depressing Emax. The Arunlakshana and Schild plots of the data were linear and had slopes not significantly different from unity. The pA2 estimates obtained were 9.17 (9.13-9.21) for prazosin, 8.58 (8.07-9.09) for phentolamine, 6.70 (6.44-6.98) for yohimbine and 7.05 (6.81-7.30) for corynanthine. Clonidine had a pA2 value of 6.60 (6.55-6.67) against phenylephrine. On the basis of results obtained with antagonists, the postjunctional alpha-adrenoceptors of the rat seminal vesicle could be firmly placed in the gross category of "alpha 1".(ABSTRACT TRUNCATED AT 250 WORDS)