p-Aminobenzoic acid and p-aminosalicylic acid-N-acetyltransferase activities were determined in 105,000 X g liver cytosols from homozygous rapid acetylator (Bio. 4.24, Bio. 41.56 and Bio. 65.67) and homozygous slow acetylator (Bio. 1.5) inbred hamsters. Liver N-acetyltransferase activities were substantially higher in rapid as opposed to slow acetylator hamsters. Genetic crosses yielded F1 generation progeny with unimodal distributions of N-acetyltransferase activity intermediate between those of the rapid and slow acetylator parental strains, and F2 generation progeny with trimodal distributions (low/intermediate/high) of N-acetyltransferase activity not significantly different from a ratio of 1/2/1. Backcross matings yielded bimodal distributions of N-acetyltransferase activity in a ratio (intermediate/parental) not significantly different from 1/1. Each of these findings is consistent with simple autosomal Mendelian inheritance of two codominant alleles at a single gene locus.