The melanophores of Phoxinus phoxinus and Salmo gairdnerii remain expanded during an initial phase in development. Treatment with thiourea prolongs this phase, which is associated with a hypothyroid condition. The young fry show a difference from the adults in their rate of response to environmental changes. It is suggested that this is due to an internal change, arising as a result of metabolic stress and causing an endocrine imbalance. No effect of thiourea on the melanophores of isolated skin strips, or on the adults of Phoxinus could be demonstrated.