The South African clawed toad, Xenopus Iœvis, ovulates and extrudes its eggs into the water after injection of gonadotropic extracts (Hogben, 1930). Xenopus has been shown to be a most satisfactory test animal for the detection of chorionic gonadotropin in the urine of pregnant women, provided the toad is in good condition. In the last ten years much work has been done on the husbandry and breeding of these animals and good laboratory conditions specified (Alexander and Bellerby, 1938; Landgrebe, 1939; Landgrebe and Purser, 1941; Gasche, 1943; Landgrebe and Samson, 1944). This toad has many advantages over other test animals. It lives indefinitely in the laboratory; the author has kept some animals for over twelve years, and they still respond to injection of gonadotropic extracts by ovulation and extrusion of eggs into the water. Recognition of this response does not necessitate killing the animal. Since Xenopus never ovulates spontaneously under normal laboratory conditions and does not extrude all its eggs simultaneously after an injection of gonadotropic extract, it can be used repeatedly. Each toad can be used at least twenty-four times under the conditions specified. The animals can be kept in good condition without much demand on laboratory routine. They need feeding and cleaning only once a week. The toads are very viable. Apart from the few deaths which often occur soon after a new stock arrives in this country, the writer has lost only eight animals in the last three years, during which time the average number kept was 500 and the number of test injections exceeded 5000. If kept at 22° C. the normal animal in good condition ovulates and extrudes eggs into the water within 24 hours after the single large dose of gonadotropic substance used in routine pregnancy diagnosis. These advantages, and the possibility of realising a simple 24-hour quantitative assay of chorionic gonadotropin, prompted an investigation into the use of the toad as a test animal for this purpose. Necessary preliminaries include inquiry into the effect on sensitivity of (1) temperature, (2) light, (3) weight of animal, (4) repeated use, and (5) route of injection. These points have been investigated with a view to the discovery of the most important variables. All the animals used have been in captivity for many years, and had previously been used many times for routine Hogben tests for pregnancy.