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Calcineurin inhibitors are important immunosuppressive agents, but cause nephrotoxicity.Instantaneous intra-renal transplant hemodynamics were assessed in 22 patients using quantitative cineloop color Doppler imaging after dosing with microemulsion cyclosporine (CSA) or tacrolimus (TAC).CSA dosing resulted in renal hypoperfusion, with a mean relative reduction of 43%±20% (range 22–76%) in maximal fractional area (MFA) of color pixels to nadir, compared to baseline. The mean effect occurred 1.1±0.9 hr (median 1 hr) after CSA dosing and was abrogated by calcium channel blockers (P <0.05). The main renal artery velocities, resistive index and small vessel perfusion were unchanged, suggestive of medium-sized arteries mediated vasoconstriction. In contrast, TAC did not alter renal vascularity (2.3±4.0% absolute reduction of MFA color pixels vs. 10.7±6.5% with CSA, P <0.01).CSA, but not TAC, induces phasic hypoperfusion of variable severity within small to medium sized intra-renal arteries soon after dosing, mitigated by calcium channel blockade.