Recently, in addition to conventional neurotransmitters such as acetylcholine, dopamine, glycine and gamma-aminobutyric acid (GABA), putative neuroactive peptide transmitters have been localized to specific retinal amacrine cells. In particular, opiate receptors 2,3, assayable enkephalin immunoreactivity and enkephalin-immunoreactive neurones 1,5 have been described in avian and mammalian retinae. However, little physiological evidence has been obtained for the involvement of neuropeptides in retinal function. Here we report that exogenous opiates affect both the release of GABA from GABAergic amacrine cells and the firing patterns of ganglion cells in the goldfish retina. Our results show that the output of the retina is modulated by an opiate system whose neural organization and pharmacological properties resemble those described elsewhere in the vertebrate central nervous system.