The requirement of the apparatus was to measure the retention of ⁴⁷Ca by greyhounds as part of a wider study of calcium metabolism in human metabolic bone disease, and, in dogs of this size, variation in counting efficiency due to changing distribution of radioactivity presents a considerably greater problem. However, in the studies, up to 50 mu Ci ⁴⁷Ca had to be administered to permit long term measurements of plasma radioactivity, so it was possible to overcome this problem by using a sensitive large area detector 60 cm from the dog, which was placed in a specially constructed counting chamber.