The effects of the close arterial infusion of histamine upon the microcirculation of facial and nasal tissues were examined in dogs. Blood flow through arteriovenous anastomoses (AVA flow), capillaries (CAP flow) and collaterals (COL flow) were determined by electromagnetic flowmetry and the tracer-microsphere technique following an infusion of histamine at doses ranging from 0.5 to 50 nmol/min. Low doses of histamine (0.5-5.0 nmol/min) resulted in an increase in blood flow through the ipsilateral internal maxillary artery (IMA), which could be mainly attributed to a significant elevation of the CAP flow. A concomitant marked increase in AVA flow was observed only after the administration of higher doses (20-50 nmol/min). Significant changes in systemic blood pressure, heart rate and cardiac output occurred only after the infusion of histamine at doses of 20 and 50 nmol/min. Significant increases in the CAP flow of tissues with relatively low perfusion were observed after the infusion of histamine at lower doses. The CAP flow of structures which play an essential role in conditioning the inspired air and exhibit high perfusion rates under control conditions exhibited significant increases only after the administration of higher doses. The present experiments provide direct evidence for a dose-dependent vasodilatory effect of histamine on different microcirculatory compartments of cutaneous and mucosal vascular beds supplied by the IMA in the dog. The results indicate that at low blood histamine levels, an increase in CAP flow predominates, and at higher doses, both elevated CAP flow and elevated AVA flow contribute to the vasodilatory response to histamine. The investigation suggests that histamine is not involved directly in a selective dilation of the AVA of the facial and nasal vasculature which is characteristic for cold-induced vasodilation.