Intravascular aggregation in response to ADP, thrombin, arachidonate and collagen has been studied in the rat with a view to throwing more light on the validity, the reproducibility and physiology of the model. The radioisotopic technique of labelling platelets with indium-111 oxine was employed. The model is minimally invasive and involves collecting blood from donor rats, separating and labelling their platelets with indium-111 oxine and assessing the accumulation of platelets in the lungs of recipient rats following the intravascular administration of aggregating agents. Of the parameters evaluated, percentage peak increase in radiolabelled platelet count and area under the curve are good parameters of expressing aggregatory responses. Of the visceral organs evaluated, the lung is the most important organ for assessing platelet accumulation. Of the vascular routes examined, no aggregatory response occurred in the lungs when ADP was injected via any of the intra-arterial routes, and of the intravenous routes, injection via the tail vein gave the highest response. The results of this investigation provide some more detailed technical information to take note of when studying platelet aggregation in vivo by this model. The results also highlight the physiological phenomenon involved.