Inhibition of K+ channels might mediate renal vasoconstriction. As inhibition of a single K+ channel caused only minor or absent renal vasoconstriction in vivo in rats we hypothesized that several classes of K+ channel must be blocked to attain sufficient depolarization to elicit vasoconstriction. We measured renal blood flow (RBF) in anesthetized Sprague Dawley rats using an ultrasonic flow probe. Test agents were infused directly into the renal artery. Inhibition of BKCa and Kir channels (with TEA and Ba2+, respectively) caused a small or transient drop in RBF (to 93 ± 2 % and 95 ± 1 % of baseline, respectively). KATP, SKCa or Kv channel blockade (with glibenclamide, apamin and 4-aminopyridine, respectively) had no effect. In contrast, a cocktail of all blockers caused a massive reduction of RBF (to 15 ± 10 % of baseline after 4 min). The response to the cocktail exhibited desensitization as the RBF response after the second stimulation amounted to 53 ± 8 %. The L-type channel blocker nifedipine abolished this reduction (94 ± 3 % of baseline NS vs. baseline; NS vs. baseline after 4 min). Furthermore, the putative T-type channel blocker mibefradil attenuated the response to the cocktail (86 ± 4 % of baseline after 4 min; p< 0,05 vs. baseline) at a calculated plasma concentration of 0.1 mM. Combined BKCa, KATP, SKCa and Kir channel stimulation (with NS1609, pinacidil, NS309 and K+, respectively) vasodilated the kidney (RBF 106 ± 1 % of baseline after 4 min). To conclude, inhibition of a single K+ channel causes no or minor renal vasoconstriction. It is possible that a mechanism, which affects several K+ channels, plays a role in the regulation of renal hemodynamics. Our results also suggest that the renal vasoconstriction elicited by K+ channel blockade is mediated by nifedipine sensitive Ca2+ channels and possibly to some extent by T- type channels. Furthermore, generalized opening of K+ channels have only a limited potential to cause renal vasodilation.