We have examined the plasma exudation response of inflammation in guinea pig skin by a noninvasive method and have evaluated the influence of vasodilatation. Indium radionuclides have been used to label plasma and blood and conversion electrons have been detected by an external detector. Transferrin (79,600 Da) was labeled by 111In or 113mIn in vivo and red blood cells were labeled by 111In in vitro. These tracers were given to separate groups of anesthetized guinea pigs and baseline activities were recorded from shaved skin surface areas. Skin prick tests with histamine and saline were performed and time-activity curves were recorded. The measurements with 111In-transferrin and 111In-labeled red blood cells demonstrated that histamine produced dose-dependent accumulation of plasma (up to a 6.5-fold increase) and blood (up to a 2.0-fold increase) in the skin. Hence, about one-third of accumulation of plasma induced by histamine may be explained by vasodilatation. With 113mIn-transferrin as plasma tracer greater effects of histamine were recorded, probably reflecting that the measurements also included deeper sections of the skin. We conclude that the intensity of accumulation of plasma in skin inflammation can be monitored by external detection of conversion electrons from 111In- and 113mIn-transferrin, and that the influence of vasodilatation can be estimated by detection of 111In-labeled red blood cells.