We used genistein (Gen) and tyrphostin 23 (Tyr-23) to evaluate the importance of tyrosine phosphorylation in norepinephrine (NE)-induced changes in intracellular free calcium concentration ([Ca²⁺]_i) in rat afferent arterioles. [Ca²⁺]_i was measured in microdissected arterioles utilizing ratiometric photometry of fura-2 fluorescence. The control [Ca²⁺]_i response to NE (1 µM) consisted of a rapid initial peak followed by a plateau phase sustained above baseline. Pretreatment with the tyrosine kinase inhibitor Tyr-23 (50 µM, 10 min) caused a slow 40% increase in baseline [Ca²⁺]_i. Tyr-23 attenuated peak and plateau responses to NE, both by ~70 %. In the absence of extracellular Ca²⁺ (0 Ca), Tyr-23 reduced the immediate [Ca²⁺]_i response to NE by ~ 60 %, indicative of mobilization of internal stores, and abolished the plateau phase. In other arterioles, the [Ca²⁺]_i response to depolarization induced by KCl (50 mM) was not attenuated by Tyr-23, indicating no direct effect on L-type Ca⁺ channels activated by depolarization. The Ca²⁺ -channel blocker nifedipine (1µM) inhibited the NE response by ~ 50 %; the effects of nifedipine and Tyr-23 were not additive. Nifedipine had no inhibitory effect after Tyr-23 pretreatment, indicating Tyr-23 inhibition of Ca²⁺ entry. Another tyrosine kinase inhibitor, Gen (5 and 50 µM), did not affect baseline [Ca²⁺]_i. High dose Gen inhibited the peak and plateau response to NE by 87 and 75 %, respectively; low dose Gen attenuated both responses by ~20 %. In 0 Ca, Gen (50 µM) abolished the immediate [Ca²⁺]_i mobilization response. Combined nifedipine and Gen (50 µM) inhibited the rapid NE response by ~90 % in the presence of extracellular Ca²⁺. Gen (50 µM) also inhibited by 60% the [Ca²⁺]_i response to 50 mM KCl, indicating a direct interaction with voltage-sensitive, L-type Ca²⁺ entry channels. These results indicate that tyrosine phosphorylation is an important link in the chain of events leading to -adrenoceptor-induced Ca²⁺ recruitment (both entry and release) in afferent arteriolar smooth muscle cells. Furthermore, different blockers of tyrosine kinase appear to have different modes of action in renal microvessels.