Abstract Male mice of 7 different strains were injected i.p. with 400 mg/kg of butylated hydroxytoluene (BHT). 2 and 4 days later, the incorporation of thymidine into pulmonary DNA was significantly increased in all treated animals and this was accompanied by an increase in lung weight and pulmonary DNA. Thymidine kinase activity and DNA polymerase activity were enhanced in the lungs of BHT-treated animals and maximum activity of these enzymes appeared to precede maximum thymidine incorporation by 24 h. 3 days after BHT a good correlation was found between administered dose and thymidine kinase activity. Measuring the activity of this enzyme might serve as a convenient biochemical marker to follow and to quantitate BHT-produced cell proliferation in lung. The concentrations of cyclic AMP and the activity of adenylate cyclase were not altered by BHT on days 1–9 after administration. BHT produced also some dose-dependent, time-dependent increases in the activities of pulmonary 5'-nucleotidase and glucose6-phosphate dehydrogenase (G6PDH), but had little effect on isocitric dehydrogenase (ICDH), pyruvate kinase (PK) and lactic dehydrogenase (LDH).