The phyllosilicate clay, hydrated sodium calcium aluminosilicate (HSCAS), has been shown to prevent aflatoxicosis in farm animals by reducing the bioavailability of aflatoxin. The present study was designed to determine the effects of HSCAS on the metabolism of aflatoxin B 1 (afb 1) in an aflatoxin-sensitive species. Male Fischer-344 rats were orally dosed with 1.0, 0.5, 0.25 and 0.125 mg AFB 1/kg body weight alone and in combination with 0.5% HSCAS. Urine samples were collected after 6, 24, 36, and 48 h. Aflatoxin M 1 (AFP 1) and aflatoxin P 1 (AFP 1) were detected in most urine samples, with or without HSCAS. AFM 1 was found to be the major metabolite. Metabolite concentrations were significantly decreased in the presence of HSCAS, and more importantly, no additional metabolites were detected. Our results suggest that the AFB 1-HSCAS complex was not significantly dissociated in vivo, and support earlier findings that HSCAS tightly binds aflatoxin.