The effect of VIP was studied in muscle strips isolated from sheep rumen. This neuropeptide caused a concentration-dependent (10-9-5.10-8 mol/l) reduction of acetylcholine-induced (5.5. 10-6mol/l) isotonic contraction of the muscle strips. The inhibitory effect of VIP was not affected by incubation of the preparations with phentolamine, propranolol or tetrodotoxin; the relaxation exerted by VIP is therefore myogenic. Because nerve fibres containing VIP have recently been identified in ruminal smooth muscle, VIP probably acts on ruminal smooth muscles as an inhibitory transmitter.

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