Two different subpopulations of adrenergic alpha-receptors -- "high-affinity" (pD2 less than 8) and "low-affinity" (pD2 less than 6) ones -- are involved in alpha-adrenergic contractions of the rat spleen muscle evoked by noradrenaline in vitro. Only the "high-affinity" subpopulation is agonistically influenced by the alpha 1-mimetic phenylephrine (pD2 cca 6, pKA cca 4.8), antagonistically influenced by prazosin (pA2 cca 9); it has a substantial receptor reserve for noradrenaline and a somewhat lower reserve for phenylephrine, it can be desensitized by these mimetics and it requires the presence of exogenous Ca2+ in the medium for evoking the contraction. -- Only the "low-affinity" subpopulation is agonistically influenced by the alpha 2-mimetic UK-14304. Both subpopulations are activated by noradrenaline and competitively blocked by yohimbine. The effect of neither subpopulation is influenced by nifedipine. According to the new classification (11) the "high-affinity" receptors can be described as alpha 1A, the "low-affinity" ones as alpha 2A. Contrarily to current data, the peculiar feature of this second subpopulation is a low affinity for yohimbine (pA2 less than 7) and a certain extent of blockability by phenoxybenzamine.