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It is known that the flow of blood through an extremity is decreased by injection of serotonin (5-hydroxytryptamine [5-HT]) into its arterial blood supply. 10,12,14 This is assumed to be due to narrowing caused by spasm of the vascular conduits. Serotonin also causes marked aggregation of platelets, leukocytes, and erythrocytes both in vitro and in vivo. 17 This has been observed in vivo to be accompanied by a marked slowing of the circulation in the conjunctival blood vessels and, after repeated injections of serotonin, by significant reductions in the hematocrit. These changes could be due to, or contributed to by, microembolism of the smaller blood vessels by aggregated blood cells which remain sequestered in the capillary beds. In the present paper this problem is explored, and the response of the cerebral circulation to intravascular injections of serotonin is reported. Material and Methods Adult mongrel dogs weighing 14.6 ± 3.2 kg were

The use of time concentration curves made with intravenously injected radioisotope and externally placed γ-ray detectors for studying cerebral circulation was the subject of a previous report. 4 A method was described which uses such curves made with one detector placed against the head and another over the heart, and which enables the expression, in arbitrary units, of the volume of circulating blood within the range of the head detector. A theoretical development of the quantitative dimensions of the curves was presented and used as the basis for the method. This communication concerns the modification and elaboration of the method to permit the expression, also in arbitrary units, of the blood flow rate through the region monitored by the head detector. The theoretical background for this is contained in the Appendix of this paper. In its present state of development, therefore, the method offers a means for measuring blood flow