MEGESTROL acetate (17α-acetoxy-6-methylpregna-4,6-diene-3,20-dione) (BDH 1298) and its C16-methylene analogue melengestrol acetate (17α-acetoxy-6-methyl-16-methylenepregna-4,6-diene-3,20-dione) (BDH 1921) are steroids with high progestational and anti-ovulatory activity and with little or no oestrogenic and androgenic properties. Megestrol acetate is used in women for oral contraception1,2 and both megestrol acetate and melengestrol acetate are being investigated for possible use in the treatment of certain hormone-dependent tumours3,4. During a study of the activity of these drugs against an oestrogen-dependent renal adenocarcinoma5 in the hamster, animals frequently died of rupture of the aorta. Some ruptures occurred spontaneously and others during routine handling of the animals. This phenomenon was investigated further in non-tumour bearing animals.