The metabolism of N-nitrosodimethylamine (NDMA) during a 5- to 10-day process of 20% protein repletion was investigated, in vitro and in vivo, in male weanling albino Wistar rats acutely and chronically deprived of protein by feeding, respectively, diets containing no protein or 3.5% for 7 days and one with 3.5% protein for 35 days. Results of experiments using the 5-day rehabilitated animals showed that rate of NDMA demethylation by liver microsomal preparation was 3-fold higher in the chronically protein-dependent animals but comparable between control animals and the more acutely deprived species. In addition, urinary excretion of an oral dose of 40 mg NDMA/kg was significantly less in the chronically protein-dependent animals, which also cleared the blood of an intravenous dose of 5 mg NDMA/kg more rapidly. NDMA metabolism was comparable in all animal groups after 10 days of protein replection.