The goose pineal gland rhythmically produces two 5-methoxyindole compounds, namely melatonin and 5-methoxytryptophol. Melatonin concentrations were high at night and low during the day, while in contrast 5-methoxytryptophol levels were markedly higher during the day compared to the night-time values. Rhythmic oscillations in melatonin content, with high night-time values, have also been found in plasma and the retina of goose. The pineal and retinal melatonin rhythm mirrored oscillations in the activity of serotonin N-acetyltransferase (AA-NAT; the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway). Acute exposure of geese to light at night markedly decreased melatonin levels in the pineal, plasma, and retina. In addition, this light exposure resulted in a significant increase in pineal 5-methoxytryptophol content. Our results demonstrate, for the first time, the ability of the goose pineal gland and retina to synthesise melatonin and 5-methoxytryptophol in a rhythmic manner.