The concentrations of adrenaline and noradrenaline in the brains of chickens (Gallus domesticus) have been determined using two different methods of fluorimetric analysis.2. The concentrations of adrenaline, noradrenaline and also dopamine were found to increase with age, as did the relative amount of adrenaline present in the chicken brain.3. It was observed that the differential assay of adrenaline by the trihydroxyindole method often underestimated the amount of adrenaline present.