Plasma exudation and vasodilatation are key microvascular features of acute inflammation. Exudation and vasodilatation responses in the weal area after skin prick testing with histamine are essentially completed within 30 min. There is evidence to suggest that vasodilatation lasts considerably longer after provocation with allergen, but there is no information on the duration of plasma exudation. The purpose of this study was to measure the time course of the microvascular inflammatory response in the skin after histamine and allergen provocation. Skin prick tests were performed with histamine, allergen (ovalbumin) or saline (control) on guinea-pigs which were shaved on their backs. Radioactive 113mIn was used to label transferrin as a plasma tracer. Radioactivity was recorded from the superficial part of the skin by external detection of conversion electrons from the decay of 113mIn. The increase in count rate, corresponding to tracer accumulation by vasodilatation and/or plasma exudation, was used as a measure of the microvascular inflammatory response to skin prick test. The microvascular response was studied immediately and up to 30 min after provocation. The largest response to histamine and allergen occurred immediately after provocation. The exudative response then gradually declined to be absent after 25-30 min. Skin prick test with saline resulted in a small response of shorter duration. We conclude that the microvascular reaction to histamine as well as allergen provocation in guinea-pig skin has a rapid onset and a duration of approximately 30 min.