The fungal toxin, citrinin, has been implicated as a nephrotoxin. Both in vitro and in vivo studies were undertaken to examine, in a controlled laboratory setting, the effects of citrinin on overall renal function as well as on specific renal transport processes. At a dose of 70 mg/kg citrinin caused an increase in urine production and a decrease in urine osmolality in the Sprague-Dawley rat. Maximal effects were observed at 4 days after a single dose, and animals surviving for that time period regained normal renal function by 5--8 days. Animals that failed to survive first entered a period of oliguria or anuria. Fischer 344 rats did not demonstrate an unusual sensitivity to citrinin. Renal slice transport of various organic compounds, e.g., paminohippurate (PAH) and tetraethylammonium (TEA), also was inhibited with maximal effects seen at day 4. In vitro effects of citrinin on fresh renal tissue were different, in part, from the effects seen with pretreatment. No alterations of renal cortical inorganic electrolytes were observed.