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Small (< 30 g) juvenile rainbow trout (Oncorhynchus mykiss) possess retinal photoreceptor mechanisms sensitive to ultraviolet (UV), short (S), middle (M), and long (L) wavelengths. During normal development, UV photosensitivity is lost progressively until, by approx. 60 g, individuals are no longer sensitive in the UV. This shift in spectral sensitivity is associated with the disappearance of small accessory corner cones (ACCs) from the retinal photoreceptor cell mosaic: the UV cone mechanism is lost. Exposing small (< 16 g) rainbow trout to the thyroid hormone thyroxine (T₄) for a period of 6 weeks induced a precocial loss of the UV cone mechanism that was indistinguishable from the events that occur during normal development. Six weeks after termination of hormone treatment, the same individuals that had lost their UV photosensitivity after exposure to T₄ once again possessed a peak in spectral sensitivity at 360 …