This study was undertaken to ascertain if changes in the affinity to alpha 1-adrenoceptor agonists and antagonists could explain the increase in uterine artery vasoconstriction to adrenergic stimuli during the luteal phase of the estrous cycle in pigs. We also sought to determine the subtype(s) of adrenoceptor (alpha 1A and alpha 1B) in the porcine uterine artery. When phenylephrine was the agonist, uterine artery pA2 values for prazosin were 8.98, 9.04 and 9.10 in the luteal and follicular phases and in early pregnancy, respectively. The KA (dissociation constant) values for phenylephrine were 6.5, 3.7 and 4.4 (microM) in the luteal and follicular phases and in early pregnancy, respectively. The use of the putative alpha 1A-adrenoceptor WB4101 (2-[[[2-(2,6-dimethoxyphenoxy)ethyl]amino]methyl]-1,4-benzodioxane ) and the alpha 1B-adrenoceptor antagonist chloroethylclonidine indicated that both alpha 1A- and alpha 1B-adrenoceptors are present in the porcine uterine artery and that a similar magnitude of inhibition of responses to noradrenaline by each of the antagonists occurred in arteries from the luteal and follicular phases and in early pregnancy. This suggests that the alpha 1A:alpha 1B adrenoceptor ratio does not vary significantly during the estrous cycle and in early pregnancy. This study indicates that porcine uterine artery alpha 1-adrenoceptor affinity is similar in the luteal and follicular phases of the estrous cycle and in early pregnancy.