1. Irregular light falshes were played on to one eye of dark adapted rabbits for periods of 20‐80 min. The concentration of 5‐hydroxyindol‐3‐ylacetic acid (5‐HIAA) and of 5‐hydroxytryptamine (5‐HT) were estimated in left and right superior colliculi, thalami and hippocampi. 2. In rabbits exposed to such visual stimuli for 30‐60 min, there was an increase in the 5‐HIAA content of the colliculus contralateral to the stimulated retina which aberaged 17% (P = 0‐02), but no rise was seen if the exposure was shortened to 20 or prolonged to 80 min. At no time was there a difference in 5‐HIAA content between right and left thalamus or right and left hippocampus. 3. Stationary or strictly repetitive visual stimuli produced no difference between the 5‐HIAA content of left and right superior colliculus. 4. No difference in 5‐HT concentration between the two colliculi was found after any form of visual stimulation, nor did any changes occur in the other parts of the brain which were examined. 5. Irregular, prolonged visual stimualtion thus appears to activate tryptaminergic neurones terminating in the colliculi. The possibility is discussed that the 5‐HT released at this site might act as a brake to neuronal activity under conditions when habituation to the stimuli is not yet complete.