AbstractWhen 6‐OH dopamine (6‐OHDA) was injected into the body cavity of eels (Anguilla anguillaL.), which had been kept on a light background with continuous artificial illumination or a natural day‐night regime in May and June, evident but heterogeneous chromatic reactions were induced. After peripheral drug effects during the first 6 hours, the eels became darker for 2 or 3 days, whereupon the melanophore index (MI), in most cases, decreased. Darkening was accompanied by significant nuclear hypertrophy and moderate degranulation of the MSH (melanophore‐stimulating hormone) cells. Probably only some of the adrenergic nerves in the neural lobe were affected. The blood supply to the brain was decreased for several days after each injection. The results support the assumption that in the eel catecholamines released from nerve endings in the neural lobe inhibit the release of MSH from intermedia cells by their own action as an inhibiting factor, or by affecting an MIF (MSH‐release inhibiting factor) and/or an MRF (MSH‐releas