To record intraocular pressure (IOP) of three different dog breeds following administration of one drop of 1% tropicamide. Three dog breeds -- Golden Retrievers (n = 20), Siberian Huskies (n = 20) and English Cocker Spaniels (n = 36) -- were studied. IOPs were measured using a Tonopen following corneal anesthesia with a single drop of 0.5% proxymetacaine. A drop of 0.5% tropicamide was then administered bilaterally and a second IOP measurement was taken 30 min later (postdilation). The difference between the two measurements was considered as the effect of mydriasis on IOP. Dogs had an average IOP of 14.9 +/- 3.2 mmHg, with 95% confidence limits ranging from 8 to 22 mmHg. There were significant differences between breeds (P < 0.006) with Siberian Huskies having higher IOPs (17.2 +/- 3.7 mmHg) than the other breeds (Spaniels: 14.2 +/- 2.8 mmHg, P < 0.01; Retrievers: 14 +/- 1.9 mmHg, P < 0.001). The majority (60%) of dogs displayed 5 mmHg or less in IOP change postmydriasis. Siberian Huskies showed the highest IOP levels, and also had the greatest variability with dilation. Interbreed variability in effect of tropicamide of canine IOP is evident.