The effect of nicotine on the meiotic maturation of cultured hamster oocytes was investigated. Oocytes were cultured for up to 24 h and meiotic status was scored cytogenetically from chromatin spreads. The highest concentration of nicotine tested (5 mM) caused marked perturbations at both the first and second meiotic divisions which resulted in degenerating blobs of chromatin at 24 h. The most pronounced of these perturbations was either blockage at Metaphase I or disruption of homologue segregation to result in two groups of bivalents formed at Anaphase I. Concentrations of nicotine at or below 0.5 mM did not adversely affect the meiotic process. These data show, therefore, that nicotine significantly interferes with oocyte meiotic maturation only at concentrations far in excess of 0.1 to 1.0 microM, the concentration range of nicotine anticipated to be present in the blood of an "average" smoker. Thus, we conclude that the documented reduced fertility of women who smoke cigarettes is unlikely to be due to a direct effect of nicotine on oocyte meiotic maturation.