Second order neurons within intact isolated pineal glands of the rainbow trout were explored by extracellular recordings to investigate modulatory effects of putative intrapineal neurotransmitters. Acetylcholine, dopamine, and norepinephrine were found to increase ganglion cell activity in a majority of cells tested. The excitatory effects of acetylcholine, dopamine, and norepinephrine were mimicked by muscarinic, dopamine D2, and beta-adrenergic receptor agonists and significantly increased with the applied light intensity, resulting in an attenuation of the ganglion cell response to light. GABA decreased discharge activity in most cells tested. This effect, which could be mimicked with the GABAA receptor agonist piperidine, was independent from the adaptive status. Acetylcholine and GABA were still active if applied during synaptic blockade with low Ca++ high Mg(++)-perfusion medium, whereas dopamine and norepinephrine exhibited no effects if applied during synaptic blockade, suggesting a differential cellular distribution of neurotransmitter receptors in the trout pineal gland. These data demonstrate that ganglion cell activity in the trout pineal gland is under the influence of several neurotransmitters, including acetylcholine, dopamine, norepinephrine, and GABA, which is in contrast to the originally proposed simple bineuronal transduction pathway from photoreceptors onto ganglion cells. Since the above-mentioned neurotransmitters are believed to be released from pineal interneurons, we may conclude that ganglion cell activity in the teleost pineal gland is, similarly to the retina, the product of photoreceptor signals and a modulatory active interneuronal system.