The metabolism of imipramine was investigated by the incubation of C-14 labelled compound in Krebs-Ringer bicarbonate buffer with microsomes of small intestine and of liver from guinea pigs. Imipramine and its metabolites were extracted with chloroform, separated by TLC and determined quantitatively by direct scanning with a TLC Linear Analyzer. With intestinal microsomes, the following metabolites could be identified: DMI, 2-OH-IMP, IMP-N-oxide. DMI is the main metabolite. The same metabolites appeared after incubation with liver microsomes, but the proportions were different: the N-oxide formation was predominant, followed by N-demethylation and hydroxylation. The formation of DMI and 2-OH-IMP seems to follow Michaelis-Menten kinetics, both in assays with intestinal and with liver microsomes. The liver/intestine ratio of DMI and 2-OH-IMP formation is proportional to the cytochrome P-450 ratio in the microsomes, in contrast to N-oxide formation.