Micronucleus induction by the diarrhetic shellfish toxin okadaic acid (OA) was investigated in two intestinal models, cultured human Caco-2 cells and colon epithelial cells of mice treated in vivo. Exposure to OA for 4 and 24 h induced dose-responsive increases in the frequency of micronucleated Caco-2 cells; the minimum OA doses increasing micronucleus frequency were 20 nM for the 4 h treatment and 5 nM for the 24 h treatment. OA treatment of Caco-2 cells also resulted in dose- and time-dependent increases in mitotic arrest and multinucleated cells. Two experiments were conducted in which mice were treated with single oral gavages of 435-610 and 115-1341 microg/kg OA. In the first experiment, samples were taken 24 h after the treatment, and the frequencies of both micronucleated and mitotic gut cells were increased after treatment with 525 microg/kg OA. In the second experiment, no increases in micronucleus frequency were detected at 24, 36, or 48 h following OA doses of 230 and 115 microg/kg; however, an increase in the mitotic index was observed 36 h after a gavage with 115 microg/kg OA. In this experiment, doses higher than 230 microg/kg were rapidly lethal to the mice. Immunohistology with monoclonal OA antibodies showed that OA was distributed into the liver at all the sampling times and in the small intestine at 24 and 36 h; OA was not detected in the colon. In addition, the TUNEL assay indicated that OA induced apoptosis in mouse ileum, liver, and kidney. The results of our investigations suggest that OA is aneugenic in Caco-2 cells, whereas the in vivo data were inconclusive. Further studies should be performed in mice using intragastric doses of 230-525 microg/kg OA. Moreover, the apoptosis and cell proliferation results indicate that OA can reach organs other than colon, indicating further evaluation of the genotoxic potential of OA in these organs is warranted.