The effect of vasodilation on simultaneously measured interstitial fluid pressure (IFP, micropuncture) and blood flow (laser-Doppler) in the low compliant pulpal connective tissue was investigated in 10 cats. Vasodilation was induced by electrical stimulation of the tooth after pretreatment with the sympathetic blocker guanethidine. Visualization of the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP) was performed using immunocytochemistry. The study was designed to answer the following questions. (1) Does vasodilation promptly increase IFP in low compliant tissues? (2) Does an increase in IFP counteract the blood flow increase? (3) Does repeated electrical stimulation cause reduced staining of CGRP- and SP-immunoreactive nerve fibers in the dental pulp? Electrical stimulation resulted consistently in a nearly synchronous increase in both blood flow and IFP. IFP was nearly doubled, from 6.3 +/- 0.18 mm Hg in control to 11.7 +/- 0.44 mm Hg, whereas blood flow increased by 28%. However, despite continued vasodilation the IFP fell to control level, or even lower, within 1-5 min. The results indicate that the increased IFP will promote fluid absorption into the blood, counteracting a further IFP increase in low compliant tissues during vasodilation. Accordingly, transmural pressure is only transitorily reduced and compression of vessels does not take place. There was considerably less CGRP- and SP-immunoreactive fibers in the stimulated teeth than in the contralateral controls, suggesting that the vasodilation was caused by liberation of these sensory neuropeptides.