This study set out to identify the neurotransmitters involved in autonomic vasodilatation of the guinea pig uterine artery. Non-noradrenergic, paracervical neurons supplying this artery contain at least four neuropeptides: vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), dynorphin A (1-17) and somatostatin, probably in addition to acetylcholine. Transmural nerve stimulation of arterial segments precontracted with phenylephrine (3 x 10(-7) mol l-1 and treated with guanethidine (10(-6) mol l-1), produced relaxations which varied in form with the frequency of stimulation and the length of the pulse train. The relaxations were monophasic at low frequencies (< 2 Hz), and were biphasic at higher frequencies (> 5 Hz) and with longer pulse trains (> 50 pulses). Neither phase of the relaxations was reduced by hyoscine (10(-6) mol l-1), or by removal of the endothelium. The faster phase of the relaxations was selectively reduced (by 61%) during treatment with L-nitro-arginine methyl ester (L-NAME; up to 3 x 10(-5) mol l-1). This reduction was reversed by an excess of L-arginine, indicating that the fast relaxation was mediated by nitric oxide, possibly acting as a neurotransmitter. The slower phase of the neurogenic relaxation was preferentially reduced (by 43%) by the endopeptidase, trypsin (1-3 micrograms.ml-1). As VIP is the only currently identified peptide present in the paracervical neurons which causes vasodilatation, it is likely that VIP, or a closely-related peptide, is the transmitter responsible for the slow relaxation. Acetylcholine and an opioid peptide also seem to be released from the vasodilator neurons, but their effects were small, and may have been restricted to pre-synaptic sites. The slower neurogenic relaxations were inhibited by exogenous neuropeptide Y (68% reduction in amplitude), and were slightly potentiated by somatostatin (21% increase in amplitude). Therefore, endogenous stores of these peptides may also contribute to the sum effect of stimulating the paracervical vasodilator neurons. In conclusion, many different substances may act as autonomic co-transmitters from these pelvic vasodilator neurons.