The present study was undertaken to determine the temporal variation in intraocular pressure (IOP) and if this variation is under circadian clock control. The authors exposed five female and five male Beagles to four different artificial lighting regimes: 12/12 light/dark (L/D) period, 12/12 D/L period, constant light, and constant darkness. IOP was measured at 3 h intervals over a 48-h period. Statistical analysis of the data was performed by multivariate ANOVA, one-way repeated measure ANOVA and by the single cosinor method. Results showed no statistical effect of gender, eye and photoperiod on IOP values. A significant effect of time for each gender and each eye during all lighting regimes was seen, except during constant light, and also robust daily rhythmicity of IOP values in all L/D periods, except during constant light. In conclusion IOP values in the dog show a circadian rhythm and this rhythm is driven by a central pacemaker.