The effects of parasympathetic and sympathetic nerve stimulation on VIP release in relation to blood flow and secretion were studied in the cat submandibular salivary gland. Parasympathetic nerve stimulation caused a marked VIP overflow (over thousand fold increase in VIP output) into the venous effluent from the gland which was simultaneous with profuse salivation and an about 10-15 fold increase in blood flow. The VIP output was dependent on the stimulation frequency, the duration of the stimulation period as well as the glandular blood flow. At 15 Hz maximal VIP output (about 4 fmol per impulse) was obtained after about 5 min of stimulation simultaneously with the maximum of the maintained phase of vasodilation. About 50 pmol VIP was recovered in the venous effluent from the gland during 1 h of maximal nerve stimulation. The VIP output after 1 h, was only about 20% of maximal, however, suggesting that the storage reserves and/or resupply of VIP might be running out. Under physiological conditions (frequencies less than or equal to 6 Hz) it was estimated that the axonal transport mechanism should be sufficient for replacement of VIP. At lower frequencies (2 and 6 Hz) the VIP output was parallel to vasodilation and secretion, while at a high frequency (15 Hz) a much more pronounced VIP output was seen. The increased overflow at 15 Hz may either be due to an actual increase in release or to a saturation of local VIP inactivating mechanisms. When stimulated simultaneously, the parasympathetic vasodilator mechanism seemed much more potent than sympathetic vasoconstriction. Since VIP may be present in cholinergic neurons, data from the literature concerning acetylcholine release are discussed in relation to the observed VIP output.