The specific effects of N-acetyl-L-methionine on acylase I activity and of both N-acetyl-L-alanyl-L-alanine and N-acetyl-L-methionyl-L-alanine on N-acylpeptide hydrolase activity were investigated in primary rat hepatocyte cultures. Each of the above two substrates is known to be much more rapidly hydrolyzed than other derivatives of the same type under optimum enzyme assay conditions. After a two-day incubation of the substrates in the presence of primary rat hepatocyte cultures, the N-acetylaminoacid was found to specifically induce an increase in the acylase I activity, whereas the two N-acetylated peptides increased the acylpeptide hydrolase activity in the soluble 100,000 x g fraction from the culture medium. No change in any of the enzyme activities could be detected during the same period of time when the medium was not supplemented with N-acetylated substrates. In addition, the acylase I activity showed a dose dependent response when the N-acetyl-L-methionine concentration increased from 10 fold to 50 fold. It is therefore suggested that the efficient hydrolysis of each type of substrate that occured in the 48 h hepatocyte cell cultures was due to the increase observed in the overall activity of the corresponding enzymes. The ratio of acylpeptide hydrolase to that of acylase I increased considerably when the medium was supplemented with N-acetylpeptides, and decreased in the presence of the N-acetylaminoacid.