Transport of procainamide, an anti-arrhythmic drug, was investigated in LLC-PK1 kidney epithelial cell line. The uptake of procainamide by LLC-PK1 monolayers cultured in plastic dishes was temperature-dependent, saturable and inhibited by organic cations such as cimetidine and N-acetylprocainamide. An aminocephalosporin antibiotic, cephalexin, also inhibited procainamide uptake, but an organic anion, p-aminohippurate, did not. The uptake of procainamide was greater at an alkaline external pH than at an acidic pH. In addition, procainamide uptake increased when intracellular pH was decreased and the uptake decreased when the intracellular pH was increased by ammonium chloride treatment, indicating the involvement of an H+/procainamide antiport system in apical membrane. The basolateral to apical flux of procainamide across LLC-PK1 monolayers cultured on permeable supports was 2.5-times larger than the apical to basolateral flux, and only the former process was inhibited by other organic cations. These findings suggest that LLC-PK1 cells can transport procainamide by the organic cation transport system and that procainamide is transported unidirectionally from basolateral to apical side across the cell monolayers.