The effects of chrysotile on DNA synthesis in the gastrointestinal tract of the rat were studied by measuring the uptake of tritium-labeled thymidine; whole stomach, small intestine, colon liver were removed, cleaned, and homogenized and the DNA was isolated and assayed for tritium label. A dose–response study indicated that 2 weeks after a 5 mg/kg dose of chrysotile, DNA synthesis was increased in the small intestine and colon and reduced in the liver; synthesis was reduced in the small intestine 2 weeks after a 500 mg/kg dose. Following a 100 mg/kg dose of chrysotile, a transient increase in DNA synthesis was noted in the stomach and small intestine at 1 and 7 days, respectively, besides increased synthesis in the colon from 28 to 63 days. These data suggest that asbestos penetrates the gastrointestinal mucosa and influences regulation of DNA synthesis in the gastrointestinal tract. ImagesFIGURE 1.