The urinary excretion of nifurpipone (I) [24632-47-1] and nitrofurantoin (II) [67-20-9] was 20-40% with a maximum in the 1st 3 hr after i.v., oral, or rectal administration, and 17-26% with a maximum in the 1st 6 hr after administration into jejunal and colon loops. A small gastric absorption was observed for II, whereas the gastric barrier was impermeable for I. The antibacterial activity of the urine of I-treated rats was due to I itself and not to I metabolites. The I metabolite 5-nitrofuraldehyde (III) [698-63-5] had in vitro antibacterial activity, whereas the urine of animals treated orally with III had no antibacterial activity.