Circumstantial evidence exists that supplementing short daylengths in cattle increases productivity by reducing activity rates (Phillips and Schofield, 1989), and stress (Leining et al, 1980). However, little is known about the preferred daylength or light intensity for cattle. Before the latter can be determined the ability of cattle to distinguish intensity differences must be determined. Two experiments are reported here: the first describes the ability of cattle to distinguish light intensity differences and the second describes their preference for supplementary light and the effect that it has on their behaviour.