Benzene and several of its metabolites were investigated for dysmorphogenic and embryotoxic effects after direct exposures of cultured whole rat conceptuses. Benzene produced no statistically significant effects at concentrations up to 1.6 mM. Inclusion with 1.6 mM benzene of an hepatic xenobiotic-biotransforming system (S9) resulted in only minor decreases in embryonic growth parameters and no detectable dysmorphogenesis. Phenol, a major benzene metabolite, also elicited only minimal embryotoxicity at 1.6 mM concentrations. However, inclusion of an S9 system with phenol resulted in significant dysmorphogenic and embryotoxic effects at concentrations as low as 0.01 mM. For phenol bioactivation, S9 from phenobarbital-induced rats was the most effective, with induction by pregnenolone-16 alpha-carbonitrile, isopropanol, Aroclor 1254, no inducer, and 3-methylcholanthrene following in order of effectiveness. Bioactivating activity resided solely in the microsomal fraction. Metabolites coeluting on HPLC with hydroquinone and catechol were the major metabolites generated from phenol by each S9 system, but no significant correlation between specific metabolite generation and embryotoxicity was apparent. Of the benzene metabolites studied, trans, trans-muconaldehyde exhibited the highest embryotoxic potency but was not detectably generated by any of the S9 systems. Hydroquinone, catechol, and benzoquinone were approximately equipotent, each producing 100% lethality at 0.1 mM. Combined additions to the culture medium of hydroquinone together with phenol resulted in greater than additive effects, indicating a possible synergistic interaction between these metabolites and suggesting that peroxidase activity may be important to the mechanism of phenol-elicited embryotoxicity.