Fluorescence microscopy and digital image analysis were used to examine the role of protein kinase C (PKC) in the control of organic anion (fluorescein, FL) transport in killifish renal proximal tubules. Phorbol ester (1-100 nM) reduced cellular and luminal accumulation of FL, and protein kinase inhibitors [staurosporine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, 10-1,000 nM] increased cellular and luminal accumulation. Phorbol ester effects were blocked by staurosporine. The increases in tissue fluorescence caused by staurosporine were blocked by p-aminohippurate, indicating that they represent increased FL transport on the organic anion system. Neither phorbol ester nor staurosporine had any effects on the cell-to-lumen transport of a fluorescent organic anion that was generated intracellularly from a nonfluorescent, uncharged precursor. Finally, studies with a fluorescent PKC inhibitor showed that phorbol ester caused PKC translocation from cytoplasm to the plasma membrane. Together, these findings indicate that renal organic anion transport is negatively correlated with PKC activity and that PKC directly or indirectly controls the basolateral step in transport.