Although progestogens protect the endometrium against excessive oestrogen-induced stimulation, they can cause adverse symptomatic and psychological effects and may have undesirable metabolic consequences. Thus, the minimum progestogen dose which results in consistent endometrial transformation should be prescribed. To define this dose for norethisterone and dl-norgestrel, 197 endometrial samples obtained from postmenopausal women receiving conjugated equine oestrogens (0.625 mg or 1.25 mg daily) with one of six doses of norethisterone (or the acetate), or one of three doses of dl-norgestrel added for the first 12 days of each calendar month were examined with the light microscope; 109 samples were also assessed by transmission electron microscopy. There was an inverse relation between the percentage of samples showing proliferative features and the progestogen dose. However, proliferative endometrium was observed in 6% of samples with the highest dose of dl-norgestrel (500 micrograms) and in 3% of samples with 2.5 mg norethisterone. Conversely, complete secretory transformation was observed in 25% of samples with the lowest dose of norethisterone (0.1 mg) and in 40% of samples with 75 micrograms dl-norgestrel. Mild atypical hyperplasia was diagnosed in four samples. There was a wide inter-patient variation in response and none of the nine progestogen dose regimens induced secretory change in every patient.