IN a previous work on the hyperfibrinogenaemic effect of several sympathomimetic amines, it was reported1 that adrenaline, when injected subcutaneously into the rat, has the highest activity in increasing blood plasma fibrinogen. We have now compared the activity of D- and L-adrenaline in relation to the hyperfibrinogenaemic response. After a preliminary experiment to find roughly the relative hyperfibrinogenaemic effect of the two forms of adrenaline, the following experiment was carried out. Forty female albino rats of 150–200 gm. body-weight were divided by randomization into four groups of ten rats each. Two groups received subcutaneously respectively 0.03 and 0.06 mgm. of adrenaline (expressed as base)/100 gm. body-weight. The other two groups were injected respectively with 1 and 2 mgm. of D-adrenaline (base). 24 hr. later, the rats were bled from the aorta under pentobarbitone-sodium anaesthesia (4 mgm./100 gm. body-weight) using syringes coated with ‘Dry Film 9987’, clotting being prevented by the addition of 0.01 ml. of 30 per cent potassium oxalate per ml. of blood; the D- and L-adrenaline were Winthrop Research Products. The fibrinogen was determined by the classical technique of Cullen and Van Slyke, with ashing by the method of Campbell and Hanna2, distillation and titration of ammonia. The results are summarized in Table 1. The relative potency of D-adrenaline as compared to L-adrenaline was calculated by standard statistical methods for a four-point assay. Our results indicate that the dextro-compound has 2.8 per cent of the activity of the laevo-compound, with the exact fiducial limits of 1.8 and 5.9 per cent. This potency ratio of 1 : 36, obtained for the hyperfibrinogenaemic effect of the dextro-compound in relation to the laevo-compound, is lower than the ratio 1 : 20 obtained by Tainter3 for the pressor response in the cat. It is, however, comparable to the 1 : 30 to 1 : 40 ratio found by Fromherz4 for the pressor response in dogs.