Abstract4‐Oxo‐all‐trans‐retinoic acid, 4‐oxo‐13‐cis‐retinoic acid and all‐trans‐retinoyl‐β‐glucuronide were intraamniotically microinjected in rat embryos on day 10 of gestation and cultured until day 11.5. A comparison of the concentration‐effect relationships showed that the dysmorphogenic effects produced by these metabolites were qualitatively similar to those of parent all‐trans‐retinoic acid. Compared with all‐trans‐retinoic acid (300 ng/ml), the dysmorphogenic effects were elicited by a 2‐fold higher concentration of 4‐oxo‐all‐trans‐retinoic acid, an approximately 10‐fold higher concentration of 4‐oxo‐13‐cis‐retinoic acid and a 16‐fold higher concentration of all‐trans‐retinoyl‐β‐glucuronide. A surplus of uridine 5 ′‐diphospho‐glucuronic acid, microinjected together with 300 ng/ml all‐trans‐retinoic acid, decreased the observed embryotoxicity of all‐trans‐retinoic acid, suggesting the possibility of glucuronidation in tissues of the conceptus per se. The results of the study provide further support for the hypothesis that 4‐oxo‐all‐trans‐retinoic acid and all‐trans‐retinoic acid are, in c