Vasopressin (VP) acts as a neurotransmitter or a neuromodulator on noradrenergic locus coeruleus (LC) neurons by exciting them. Experiments were performed in precollicular decerebrate cats to investigate whether direct infusion of VP into the LC complex of one side produced changes in posture as well as in the gain of vestibulospinal reflexes acting on forelimb extensors. Unilateral microinjection of 0.25 microliters VP solution (10(-11) micrograms/microliters saline) into the LC complex increased the extensor rigidity in the ipsilateral limbs, while that of the contralateral limbs either remained unmodified or slightly decreased. The amplitude of modulation and thus the response gain of both the ipsilateral and the contralateral triceps brachii to roll tilt of the animal leading to stimulation of labyrinth receptors decreased (t-test, P less than 0.001 for both the ipsilateral and the contralateral responses). Moreover, a slight decrease in phase lead of the responses was observed. These findings occurred 5-10 min after the injection, were fully developed within 30 min and disappeared in about 2 h. The changes in posture as well as in the gain of vestibulospinal reflexes described above were site specific and depended upon the injected neuropeptide. They were attributed to tonic activation of presumptive noradrenergic neurons, which exert a facilitatory influence on limb extensor motoneurons either directly, by utilizing the coeruleospinal pathway, or indirectly by inhibiting the dorsal pontine reticular formation and the related medullary inhibitory reticulospinal neurons.