Abstract It has been demonstrated that after a sufficient equilibration period the principle catabolite of a tracer dose of [ 3H]pteroylglutamate is acetamidobenzoylglutamate. This catabolite was formed prior to excretion from p-aminobenzoylglutamate, a small amount of which also appears in the urine and indicates that the principle mechanism of cellular folate catabolism proceeds via cleavage of the C9N10 bond. The inability of others to demonstrate this simple catabolic mechanism resides in that they used too short a study period which shows a complex equilibration pattern in the urine or that they may have wrongly identified the other cleavage product, a pteridine, as an intact folate derivative. In addition the further metabolism of p-aminobenzoylglutamate has not been appreciated until now. Apart from the two catabolites mentioned three others have been recognized and preliminary evidence suggests that they may be pteridines which have become labelled during metabolic cleavage.