A comparative test in mice for pulmonary toxicity between butylated hydroxytoluene (2,6-di-tert.-butyl-4-methylphenol, BHT) and 2,6-di-tert.-butyl-4-[alpha, alpha, alpha-2H3]methylphenol (BHT-d3) showed a significantly lower toxic potency of the latter. The rate of in vitro BHT metabolism to 2,6-di-tert.-butyl-4-methylene-2,5-cyclohexadienone (BHT-QM) was slowed by deuterating BHT in the 4-methyl group. On the other hand, the rate of in vitro metabolism to 2,6-di-tert.-butyl-4-hydroxy-4-methyl-2,5-cyclohexadienone (BHT-OH) was increased with the deuteration. A similar isotope effect of the deuterium substitution on the in vivo metabolic rates of BHT was observed. These observations support the concept that the lung damage caused by BHT is mediated by BHT-QM. The pulmonary toxicity of 2-tert.-butyl-4-ethylphenol (4-EP) and their deuterated analogs was also compared. 2-tert.-Butyl-4-[1,1-2H2]ethylphenol (4-EP-d2) showed a significantly lower toxic potency than 4-EP, whereas 2-tert.-butyl-4-[2,2,2-2H3]ethylphenol (4-EP-d3) showed a toxic potency comparable to that of 4-EP. This result is consistent with the hypothesis that a quinone methide metabolite is responsible for the onset of lung damage produced by 4-EP as well as BHT.