Exposure to continuous illumination disrupts normal ocular development in young chicks, causing severe corneal flattening, shallow anterior chambers and progressive hyperopia ('constant light (CL) effects'). We have studied the minimum requirements of a diurnal light cycle to prevent CL effects. (1) Seven groups of 10 chicks were reared under a 0 (constant light, CL), or 1, 2, 3,4, 6, or 12/12 h (normal) light-dark cycles. It was found that CL effects were prevented if the dark period was 4 h or longer. Below 4 h, the effects were dose-dependent and inversely correlated with the amplitude of the Fourier component of illumination at 1 cycle per day (CPD). (2) Three groups of 20 chicks were exposed to 4 h of darkness distributed differently over 24 h to vary the amplitude of the Fourier component at 1 CPD. It was found that complete suppression of the CL effects required that the 4 h of darkness were given in one block and at the same time each day. Our results show that normal ocular development in the chick requires a minimum of 4 h darkness per day, provided at the same time of the day without interruption, and suggest that the light-dark cycle interacts with a linear or weakly nonlinear oscillating system.