The effects of benzo[a]pyrene, 6-hydroxymethylbenzo[a]pyrene, benzo[a]-pyrene-6-carboxaldehyde, 6-methylbenzo[a]pyrene, benzo[a]pyrene-6-carbonitrile and benzo[a]pyrene-6-carbonamide on urinary ascorbic acid excretion and hepatic tyrosine transaminase, aniline hydroxylase, aminopyrine N-demethylase and hexobarbital metabolizing activity in mice were examined. The effect of 6-bromobenzo[a]pyrene on the hepatic enzymes was examined. None of the compounds altered aminopyrine N-demethylase or hexobarbital metabolizing activity whilst all produced a statistically significant rise in aniline hydroxylase activity. The tyrosine transaminase activity was increased by 6-hydroxymethylbenzo[a]pyrene but was unaffected by the other compounds. All the compounds raised urinary ascorbic acid excretion after an initial 3 day lag. The level of excretion remained elevated for about a week or more except for 6-hydroxy-methylbenzo [a]pyrene-treated animals which showed a high raised level of excretion for 1 day followed by a drop to a lower than normal level of excretion. The significance of these observations and their relation to other work was discussed.