The effects of micro-electrophoretically applied pineal indoles, melatonin (MEL), 5-methoxytryptophol (MTL) and 5-hydroxytryptophol (HTL) and of the neurotransmitters, noradrenaline (NOR), acetylcholine (ACH) and γ-aminobutyric acid (GABA) on the electrical activity of pineal cells were studied in anaesthetized pigeons. Recordings were made from both sighted and blinded birds during both the day- and night-time. Both excitatory and inhibitory responses to the application of all substances tested were observed, while in all cases unresponsive cells were also found. The pattern of responses to MEL, MTL and to NOR varied significantly depending on whether the cells in the sighted pigeons were tested during the day or at night. This day/night rhythm of responses to MEL and to MTL also occurred in the blinded pigeons, although this was not the case for the effects of NOR. Units which were orthodromically excited by electrical stimulation of the habenular complex were also excited by application of GABA. The activity of these units was also likely to be enhanced by application of ACH. The results suggest that the pigeon pineal organ exhibits intrinsic circadian rhythmicity which does not require entrainment signals from the retina. They also indicate that the activity of pineal cells could be influenced by neurotransmitter input provided from within the gland or via its innervation.