Injection of gonadotrophin into the male British toad (Bufo bufo) is followed by the shedding of sperm, the response being of the normal quantal type (Frazer and Wohlzogen, 1951). The doseresponse curve of the toad has now been investigated more fully with chorionic gonadotrophin, which is of the luteinizing type. At the same time the response to pregnant mare's serum gonadotrophin (of the follicle-stimulating type) was also studied to see if the two curves differed greatly in either ED50 or slope. The response of the treefrog (Hyla arborea) was also investigated, since it seemed possible that this smaller species might respond to smaller doses, and so be more suitable for the estimation of relatively small amounts of gonadotrophin.The effect of chorionic gonadotrophin on the toad was also examined to see whether it altered at different times of the year under relatively standard laboratory conditions. The effect of alteration in environmental temperature was also studied. With this knowledge, the gonadotrophin excretion during pregnancy was followed in 5 cases, using a technique already developed (Frazer and Wohlzogen, 1951). METHODS Experimental Animals.-Toads and treefrogs were kept in an animal house at approximately 21.5°C. The experiments were normally conducted in a laboratory kept at about 20°C. The conditions during the temperature experiments are described later. In the seasonal work, the batches of toads were taken as far as possible through the summer at 6-week intervals, being used exactly one week after they had been obtained from the dealer. The interval between receipt and use was left to avoid using toads which might already have been shedding sperms. The isolation of the males from females for a week is normally sufficient to achieve this object, even when, on receipt from the breeding ponds, the cloacal urine of the males is swarming with spermatozoa. They were kept during this week in large cages containing moss and water, and fed on maggots and flies.The toads were from the south of England, and the treefrogs were of sp. typica from N. Italy.Gonadotrophins.-International Standard gonadotrophins were used for the dose-response curves, the luteinizing type from human pregnancy urine (UP) and the follicle-stimulating type from pregnant mares' serum (PMS); for the experiments on temperature, a crystalline preparation was made from pregnancy urine by the method of Claesson, Hogberg, Rosenberg, and Westman (1948). The gonadotrophin was always dissolved in distilled water; the injection volume was 1 ml.When gonadotrophins were being estimated in pregnancy urine, the urine was filtered and injected directly. Dilutions of urine were also used, and gonadotrophin concentrates were made from samples when necessary, either because of the toxicity of the urine or because there was only a low level of gonadotrophin in it. For this purpose, the method of Scott (1940) was used, modified by the use of Universal Indicator (B.D.H.). The technique of Frazer and Wohlzogen (1951) was used for gonadotrophin estim...
