This study was conduced to assess the daily rhythm of tear production in clinically healthy dog. For our study eight female purebred Beagles were subjected to three different Light/Dark schedules: 12/12L/D, 24/0L/D and 0/24L/D cycles. In all subjects Schirmer tear test I was performed at 4h intervals over a 24h period. A statistical significant effect of photoperiod was observed comparing the three different L/D schedules, and a statistical significant difference was observed comparing left and right eye during the 12/12L/D schedule. We demonstrated daily variation of tear production in dogs maintained under an L/D cycle. We also provided strong evidence that the rhythm of tear production is endogenously generated because it persisted in constant darkness. Although the range of excursion of the daily/circadian oscillation in STT I (about 2mm/min) is likely too narrow to be of clinical significance, it is statistically significant and may have physiological implications not yet appreciated.