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1. The microvasculature of the rabbit submandibular salivary gland has been investigated employing in vivo microscopy, blood flow measurements, latex casts, microsphere injections and examination of fixed sections of the gland.

2. Two principal microcirculations were distinguished in the living gland, one supplying the acini and the other the intralobular ducts. Parasympathetic nerve stimulation (2, 5 or 10 sec^-1) elicited different responses in each of the two microcirculations. Flow in the capillaries around the acini slowed initially before increasing. In contrast, flow in the intralobular duct capillaries increased soon after beginning stimulation.

3. In some experiments both whole gland flow and microvascular flow were monitored simultaneously. Whole gland flow increased at the same time as flow in the acinar capillaries was decreasing and as flow in the intralobular duct capillaries increased. Flow in acinar capillaries increased about 5 sec after glandular flow started to increase.

4. These observations could be explained if either the vascular beds of the acini and the intralobular ducts were arranged in parallel or if arteriovenous anastomoses were to shunt the acinar circulation. No such anastomoses were found in latex casts made of the gland vasculature, and microspheres injected into the artery supplying the gland were not found in the venous effluent.

5. The intraglandular distribution of microspheres was measured in histological sections of the injected glands to give an estimate of the distribution of blood flow between the duct and acinar microcirculations. At rest and during maintained stimulation about 55% of the blood flow passed through the intralobular duct microcirculations, whilst during this initial 15 sec of stimulation this proportion was increased to over 70%. This finding is consistent with a parallel arrangement of the two microcirculations.

6. The conclusions drawn from these observations are that the duct and acinar microcirculations are arranged in parallel, that there are differences in the way the vasodilatation is mediated in these circulations, and that arterio-venous anastomoses play no significant role in this gland.