In vivo administration of pyridine has been shown to increase the activity and content of several forms of cytochrome P450 by transcriptional and posttranscriptional mechanisms. The effect of pyridine on CYP1A and CYP2E1 isozymes was studied in a rat hepatocyte culture model. Hepatocytes were isolated from non-induced rats and seeded onto matrigel-coated dishes and incubated in William's medium E containing 10% fetal calf serum, hormones, and essential metals. Cultures were treated with 0, 10 or 25 mM pyridine for 1-3 days and microsomes were isolated to determine catalytic activity and for immunoblot analysis, and total RNA was isolated for mRNA determinations. CYP2E1 content, CYP2E 1 mRNA, and CYP2E1 catalyzed oxidation of p-nitrophenol declined during culture to values of 3, 30 and 19% that of initial, non-cultured controls by day 3 of culture. Pyridine prevented this decline of CYP2E1 protein and activity such that 60-80% original activity remained after 3 days of culture in the presence of 25 mM pyridine. However, pyridine did not prevent the fall in CYP2E1 mRNA levels, nor did pyridine increase the content or activity of CYP2E1 above initial values of microsomes from freshly isolated hepatocytes. Pyridine increased the content of CYP1A2 and the oxidation of ethoxyresorufin 2-4 fold compared to cultures incubated without pyridine over the 3 day culture period. CYP1A1 levels, which rapidly declined, were induced and maintained in the presence of pyridine. Pyridine increased CYP1A content and activity 2-3 fold over initial values of freshly isolated hepatocytes. These increases were associated with corresponding increases in CYP1A mRNA levels. CYP1A2, but not CYP1A1, mRNA levels increased in the cultures incubated in the absence of pyridine. These results indicate that pyridine has different effects on CYP1A and CYP2E1 in this hepatocyte culture model. Pyridine appears to modulate CYP2E1 levels by posttranscriptional mechanisms as CYP2E1 activity and content were maintained in the presence of pyridine under conditions in which CYP2E1 mRNA levels declined. These mechanisms may involve increased translational efficiency of existing CYP2E1 mRNA or stabilization of CYP2E1 protein against degradation. Pyridine increased CYP1A1 and CYP1A2 content, activity and mRNA levels, either inducing CYP1A transcription or stabilizing CYP1A mRNA. Hepatocyte cultures may be a useful model to study the interaction of pyridine with P450 isozymes and their associated drug-mediated toxicity.