The intestinal oxidative metabolism of hexamethylmelamine (HMM) and pentamethylmelamine (PMM) has been studied in microsomes, isolated mucosal cells and intestinal perfused segments. (sub) Cellular systems revealed an almost equal Km (53-65 microM) and Vmax (5.6-7.0 nmol min-1 g-1 intestine) for both compounds. Detailed studies in everted intestinal perfused segments, showed that HMM is metabolized to a far greater extent than PMM (e.g. 11-times, at 80 microM substrate concentration) while PMM transport was 3 times greater than the transport of unchanged HMM. Only when perfused segments were used as an in-vitro tool was a good correlation observed between the in-vivo and in-vitro rate of intestinal metabolism of HMM and PMM. It is concluded that this is due to preservation of structural integrity of the mucosa for both absorptive and metabolic processes.