The metabolism of imipramine and desipramine was examined by using isolated rat hepatocytes. The enzyme systems having high-affinity-and-low-capacity and low-affinity-and-high-capacity kinetic properties were found to catalyze aromatic 2-hydroxylations of imipramine and desipramine, and aliphatic N-demethylation of imipramine, respectively. The Km and Vmax values for N-demethylation of imipramine (which formed desipramine) were about 5-10 and 5 times larger than those of both 2-hydroxylations respectively. A competitive inhibition between the 2-hydroxylations of imipramine and desipramine ("parallel pathway interaction") (Chiba M, Fujita S and Suzuki T, J Pharm Sci 77: 944-947, 1988), observed using liver microsomes, was found also in isolated hepatocytes. It was concluded that the characteristics of imipramine metabolism observed in liver microsomes were well reproduced in isolated rat hepatocytes.