In the present study, we examined the effect of inhibition of catalase with 3-aminotriazole (3-AT) on hydrogen peroxide (H2O2)-induced enhancement of sympathetic neurotransmission in bovine irides and on the inhibitory effect of this oxidant on norepinephrine (NE) release from human irides, in vitro. Furthermore, we investigated the effect of 3-AT on H2O2-induced attenuation of contractile responses to carbachol in the bovine isolated irides. Isolated mammalian irides were prepared for studies of [3H]NE release using the superfusion method and for contractile studies using isolated organ baths. At concentrations less than 100 microM, H2O2 had no significant effect on field-stimulated [3H]NE release from bovine or human irides. In bovine irides, 3-AT caused significant (P < 0.001) leftward shifts of concentration-response curves to H2O2 (10-300 microM). 3-AT also increased H2O2-induced attenuation of evoked [3H]NE release from human isolated irides. Low concentrations of H2O2 (< 100 microM) had no effect on carbachol contractions. However, 3-AT unmasked an inhibitory effect of low concentrations of H2O2 (3-100 microM) on carbachol-induced contractions. We conclude that inhibition of catalase causes both pre- and postjunctional responses of isolated mammalian irides to be more susceptible to oxidative stress induced by H2O2.