1. 4-Biphenylaldehyde (4-BA) and 9-anthraldehyde (9-AA) were examined as substrates for cytochrome P450 (CYPs) enzymes in rat and human. Both aldehydes were oxidized by CYPs to fluorescent carboxylic acids, which can be assayed with a high sensitivity by an easy fluorimetric method. 2. With liver microsomes from control and induced rats, the oxidation of both 9-AA and 4-BA followed simple Michaelis-Menten kinetics. Only microsomes from rats pretreated with phenobarbital (a strong inducer of P4502B1/2) could increase (about threefold) the oxidation rates (V(max)) of both aldehydes above the control values, which were 6.7+/-1.1 and 3.3+/-0.6 nmol min(-1) mg(-1) protein for 4-BA and 9-AA, respectively. On the other hand, the (K)(m)'s, which were similar for both aldehydes (about 25 micro M), did not change significantly with any inducer. The use of purified rat CYP1A1, 2E1, 2B1 and 2C11 in a reconstituted system showed that only 2B1 and 2C11 could oxidize both substrates with a high turnover. 3. In human liver microsomes, the oxidation rates of these aldehydes (1.6+/-0.2 and 0.42+/-0.1 nmol min(-1) mg(-1) protein for 4-BA and 9-AA, respectively) were lower than those of rat but with similar K(m)'s(20-26 microm). 4. The oxidation of these aldehydes was also determined with cDNA-expressed CYP1A1, 1A2, 2A6, 2B6, 2C9, 2D6, 2E1 and 3A4 and with a characterized bank of 14 human liver microsomes. In a reconstituted system, only CYP2B6, 2A6, 3A4 and with a lower turnover 2C9 oxidized both substrates. 5. Among the CYP marker activities of the 14 human samples, good correlations were only observed between CYP3A-dependent 6 beta-testosterone hydroxylase and the oxidation of 4-BA (r = 0.74) or 9-AA (r = 0.80) and between the oxidation of 4-BA versus 9-AA (r = 0.74). Weak correlations were also found between the 2B6-linked S-mephenytoin N- demethylase and the oxidation of 4-BA (r = 0.58) or 9-AA (r = 0.65). 6. Inhibition experiments revealed that the oxidation of these aldehydes was inhibited by ketoconazole, 8-methoxypsoralene and sulphophenazole, selective inhibitors for P4503A6, 2A6 and 2C9, respectively. 7. In summary, based on the use of cDNA-expressed CYPs, correlation analysis and chemical inhibition, the metabolism in human liver microsomes of these aldehydes appears primarily catalysed by CYP3A, although CYP2A6, 2B6 and 2C9 may play a role. 9-AA and particularly 4-BA, owing to the high rate of its metabolism, may be two novel useful fluorescent probe substrates for assaying CYP activities in various species.