Abstract 1. 1. The effect of adrenaline, noradrenaline, tyramine and 5-hydroxytryptamine has been studied on isolated scale chromatophores from the cuckoo wrasse, Labrus ossifagus, using normal and phenoxybenzamine (Dibenzylene)-treated scales as well as scales taken from fish injected with phenoxybenzamine or reserpine. 2. 2. Adrenaline, noradrenaline, tyramine and 5-hydroxytryptamine caused chromatophore pigment concentration in scales from normal fish. 3. 3. These amines showed a reduction or loss of concentrating activity when the scales were pretreated with phenoxybenzamine or when scales from phenoxybenzamine-injected fish were used. 4. 4. Chromatophores on scales taken from reserpine-injected fish showed normal or slightly greater response to the catecholamines while tyramine and 5-hydroxytryptamine no longer produced pigment concentration. 5. 5. The concentrating activity of tyramine could be restored by exposing the reserpinized scales to catecholamine, and noradrenaline was more effective than adrenaline in this respect.