Histological analyses of the retina of Salmo trutta post-embryos exhibit rhythmic disc shedding at 6 weeks, followed by cyclic retinomotor movements at 7 weeks. Diurnal variations in cAMP concentration, as established by a radioreceptor assay, develop simultaneously with retinomotor movements and are fully expressed at 9.5 weeks. In continuous darkness, neither retinomotor movements nor intra-ocular peaks of cAMP occur, indicating that the endogenous oscillator for retinomotor movements and cAMP levels is not yet developed.