The assay of corticotrophin was performed in mice by means of small sample analysis of free plasma corticosteroids. In this method hypophysectomy was replaced by dexamethasone pretreatment. The response was measured preferably in a single mouse weighing 20 g or more. When mice of a lower body weight were used the plasma of two randomly assigned mice was pooled. Corticosteroids (mainly corticosterone) were determined fluorometrically in 0.25 (0.20) ml samples of plasma from heparinized blood. The results show that valid corticotrophin assays can be performed in mice both by the intravenous and subcutaneous route. Compared with the adrenal ascorbic acid depletion method or the plasma corticosteroid method in the rat the assay in mice was found to be at least five times more sensitive. 40 micro-units of corticotrophin were consistently detectable. Precision was dependent on the route of administration, the mean index of precision (s/b) being 0.20 in the intravenous and 0.12 in the subcutaneous assay. The difference was due to a steeper slope of the logdose-response line after subcutaneous administration. Contrary to the findings in the rat, corticotrophin A (oxycel purified) did not differ significantly in potency estimates from subcutaneous and intravenous assays in mice, when crude corticotrophin (U. S. P. Corticotropin Reference Standard) was the basis of comparison. Accordingly results of subcutaneous assays of corticotrophin A samples in terms of the U. S. P. standard were lower in mice than in rats. The use of gelatine instead of saline as diluent in the subcutaneous assays yielded slightly but not significantly higher potency estimates (25 per cent). The interpretation of the results is that for intravenous corticotrophin assays the mouse method is comparable to the rat assay. For subcutaneous corticotrophin assays, however, the mouse method is not suitable, if crude corticotrophin (U. S. P. standard) is the basis of comparison, but if corticotrophin A (oxycel purified) is the standard of reference (e. g. the Third International Standard for Corticotrophin), the mouse method may justifiably be used. The advantages of the mouse method are increased sensitivity, precision, convenience, and economy.