The criteria for the choice of radionuclides for nonimaging studies may be quite different from those for imaging purposes. From the viewpoint of detection and measurement of the administered activity, the radionuclidic emissions need not be of suitable energy for gamma camera imaging. Detection of gamma emission with an external probe is possible either with activities administered in vivo or activities induced by neutron activation. In both instances, temporal studies can be performed with detector probes attached to scalers and recording devices, although a great degree of sophistication usually is achieved using a gamma camera and computer-aided data processing. The decision to use a camera imaging device is dependent on the suitability of the available radionuclide and the specific problem to be resolved. Nonimaging studies may be carried out with in vivo administration of radiochemicals or radiopharmaceuticals incorporating a wide range of radionuclides including pure beta emitters, beta-gamma emitters, electron capture nuclides, etc. In those instances where external body counting is not feasible or practical, the body fluids may be sampled serially and radioactivity quantitated directly. A detailed compartmental description of the distribution and metabolic turnover rate of a particular agent can be estimated in this manner. Wherever in vivo administration of activities is involved, the problem of radiation dose is of paramount importance and a careful assessment of the compartmental distribution and metabolic pathway should be sought in every instance.