This study examined whether organic anion secretion contributes to fluid accumulation in cysts in polycystic kidney disease. Clearance and micropuncture studies were done on young (7 to 16 wk old), mostly male, heterozygous Han:SPRD cystic rats and healthy control littermate rats. Heterozygous Han:SPRD rats manifest a slowly progressive autosomal dominant polycystic kidney disease that closely resembles the human disorder. Left kidney GFR (polyfructosan clearance), in microl/min per 100 g body wt, averaged 331 +/- 36 (SD) in seven healthy rats and 278 +/- 75 in seven cystic rats. The maximal rate of p-aminohippurate (PAH) secretion, in micromol/min per 100 g body wt, averaged 0.94 +/- 0.24 in healthy rats and 0.83 +/- 0.11 in cystic rats. In these young rats, there were no significant differences in GFR or the maximal rate of PAH secretion despite the presence of cystic disease. Using fluorescence microscopy, it was found that 27 of 29 proximal cysts secreted sulfonefluorescein, an organic anion transported by the PAH system. Transmission electron micrographs of superficial cysts that had secreted sulfonefluorescein demonstrated the presence of both normal-appearing and poorly differentiated proximal tubule cells. Segments of superficial proximal convoluted tubules or cysts, isolated by upstream and downstream wax blocks, failed to accumulate fluid when PAH was infused intravenously. With the stationary microperfusion technique, PAH secretion by both normal and cystic nephrons was demonstrated. It is concluded that most proximal cystic epithelia retain the ability to secrete organic anions. Secretion of organic anions, however, does not appear to contribute in any substantial way to fluid accumulation in cysts in the rat kidney.