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Optimal conditions for determining the quantitative effect of a single subcutaneous injection of progesterone in oil on the uterus:plasma concentration ratio for radioactive iodide were found to be measurement of this ratio 24 hr. after the injection of steroid in rats ovariectomized 7–10 days previously. Under these conditions there is a linear regression of response on log dose over the range of 156–2500 µg. progesterone.

Reduction of the 20-ketone group or the 4–5 double bond, introduction of 17- or 21-hydroxyl groups or of an 11-oxo group into the molecule reduces the activity in this test. Steroids with predominantly glucocorticoid, androgenic or oestrogenic activity do not produce this response. The relative potencies of a variety of synthetic steroids derived from progesterone or 19-nortestosterone have been assessed. The results obtained in this test appear to be more closely related to the ability to maintain pregnancy than are those obtained in the usual assays for progestational activity. It is suggested that the uterus:plasma ¹³¹I ratio response could be made the basis of an accurate and potentially useful assay method for steroids with progestational properties.