Colon explants from adult rats were maintained in culture for over 3 months in our laboratories with good epithelial preservation and cellular differentiation. The light and transmission electron microscopic features of rat colon mucosa during the culture period are described. In all the explants that remained viable, there was an initial phase of degeneration of the surface and crypt cells, later these areas were repopulated in one week, showing well-formed crypts, goblet cells, and ultrastructural features such as extensive lateral interdigitations, microvilli and glycocalyx--typical of colon. The effect of in vivo carcinogen pretreatment was also studied. The explant culture from control untreated animals showed good epithelial differentiation with crypts until 6 weeks. In contrast, the explants from animals pretreated with 4 weekly doses of azoxymethane consistently showed epithelial differentiation with well-formed crypts up to 13 weeks.