Norethindrone-4 beta,5 beta-epoxide was toxic to Walker cells in culture. The concentration required to produce a 50% reduction in the increase in cell numbers 72 h after exposure (ID50) was 0.05 mM. In this assay, the parent contraceptive steroid, norethindrone, was at least four times less toxic than the epoxide. 2. Norethandrolone-4 beta,5 beta-epoxide and norethynodrel-5 beta,10 beta-epoxide were as toxic as norethindrone epoxide to the Walker cells. 3. The cytotoxicity of norethindrome epoxide was dependent on the time of exposure of the cells to this compound if excess unreacted epoxide was removed by washing the cells with cysteine. The results are consistant with norethindrone epoxide causing cell death by reacting with sulphydryl groups of cellular proteins. 4. No metabolites toxic to Walker cells could be detected when the cells were incubated with norethindrone, rat liver microsomes and a NADPH generating system. 5. Cells treated with an ID50 of norethindrone epoxide for 1 h showed marked cytoplasmic vacuolation 3 hr after exposure. This vacuolation was much less marked in cells treated with an ID50 of norethindrone or in the controls. Neither group showed any nuclear abnormalities. 6. Norethindrone epoxide when given to rats in large doses (50 mg/kg) by lateral tail vein injection also caused cytoplasmic vacuolar degeneration of the liver hepatocytes, especially in the perilobular areas 3 days after dosing. When this compound was administered at a similar dose level via the hepatic portal vein massive haemorrhagic necrosis of the liver resulted. No damage to either lungs or kidneys was evident, irrespective of the route of administration.
