Optimized culture conditions were used to achieve prolonged survival of colon mucosa from conventional inbred F344 strain rats. Up to 21 days in culture, a high percentage of explants had viable mucosa with intact crypt columns and maintained active replicative DNA synthesis in the proliferative zone of the crypts. An acute 4-hour in vitro exposure to 0.5 mM of the colon-selective carcinogens 1,2-dimethylhydrazine dihydrochloride (DMH) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) produced 26 and 78% suppressions of replicative DNA synthesis, respectively, whereas the same exposure to the liver carcinogen dimethylnitrosamine produced only a 5% suppression. The thymidine-labeling index of DMH-exposed cells was lower than that of control cells at 1 day and then increased above the control values 7-21 days later. MNNG exposure produced pronounced morphologic changes in the crypt columns. MNNG-exposed cultures that were transplanted into the mammary fat pads of syngeneic female rats produced hypercellular cysts different from those formed by nonexposed fragments. These findings indicate that the acute and persistent effects of colon carcinogens can be studied directly on the target tissue in organ culture.