The effect of respiratory acidosis and alkalosis on the vasoconstriction to alpha 1- and alpha 2-adrenoceptor stimulation was studied in pithed normotensive rats. The selective alpha 1-adrenoceptor agonists (-)amidephrine, cirazoline, (+/-)erythro methoxamine, (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selective alpha 2-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selective alpha-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respective alpha 1- and alpha 2-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by the alpha 1-as well as the alpha 2-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influence alpha 1-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguished: Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)