The side effects of acetazolamide (Diamox^®) on lacrimation were measured in rats by means of the cotton-thread tear test. After a daily oral 1-mg dose (administered for five days), comparable to the dose used for adult humans on a drug-to-bodyweight basis, tear production remained unaffected but the lacrimal peroxidase secretion decreased by 60% of the baseline level. After withdrawal of acetazolamide the peroxidase secretion returned to the baseline level.