A single injection of the sex-dependent hepatocarcinogen DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1305) caused age-related DNA damage, as evaluated by alkaline sucrose gradient analysis, in the liver of female Wistar rats. DNA damage reached a maximum at 4-6 weeks of age, about the onset of sexual maturity, and decreased thereafter. In young rats (5-8 weeks of age), the amount of ZAMI 1305-induced DNA damage showed seasonal and daily differences, being higher when the molecule was administered in winter in respect to summer and in the evening in respect to the morning. In older rats (15-22 weeks of age), no seasonal and daily variations were observed.