Purpose: To investigate the effects of alpha-1 adrenergic blockers on pupil diameter (PD) in rats. Methods: Four groups, with 10 rats each, were designed to receive terazosin, tamsulosin, doxazosin, and no medication. Dilated pupil diameter (PD) measurements were performed 24 hr before, 24 hr after, and 30 days after the initiation of medication, and after 30 days of washout. The intergroup and intragroup differences in PD were evaluated using one-way ANOVA and Wilcoxon signed rank test, respectively. Results: In day 1, PD decreased in both eyes significantly only in tamsulosin and doxazosin groups, but these effects became insignificant at 30 days of treatment (p > 0.05). The control group showed no significant difference in PD (p > 0.05). PD values returned to baseline after the washout period in all groups. Conclusions: A significant reduction in PD occurred in two of the three groups with alpha-1 adrenergic blockers (tamsulosin and doxazosin), but this effect was not sustained at 30 days. Further functional and structural studies of the iris are needed to determine the clinical significance of these findings.