In normal (5mM KCI) HEPES buffer solution, BHT-920 and rauwloscine did not produce any contractile responses in dog mesenteric artery strips. However, when the preparation was bathed in 20mM KC1 HEPES buffer solution, BHT-920 and rauwloscine evoked significant contractile responses. These effects were markedly inhibited by parazosin which caused a parallel shift to the right of the concentration-response curve to BHT-920. In 45Ca uptake experiments carried out in the 20mM KC1 HEPES buffer solution BHT-920 and rauwolscine significantly increased 45Ca influxes which were reduced by prazosin. These results suggest that postsynaptic al-adrenoceptors in dog mesenteric artery mediate the contractile responses and the 45Ca influxes induced by BHT-920 and rauwloscine after partial depolarization by 20mM KC1.