1. Experiments were designed to compare effects of dexamethasone on transient (I(peak)) and sustained (I(sus)) K(+) currents in control and diabetic rat myocytes. Ventricular myocytes were isolated from control or type 1 streptozotocin (STZ)-induced diabetic male and female rats. Currents were measured using whole-cell voltage-clamp methods. 2. Incubation of cells from control males or females with 100 nM dexamethasone (5–9 h) significantly (P<0.005) augmented I(sus) (by 28–31%). I(peak) was unchanged. I(sus) augmentation was abolished by cycloheximide or cytochalasin D, but not by inhibition of protein kinases A or C. Inhibition of tyrosine kinases by genistein (but not its inactive analog genistin) prevented the increase of I(sus) by dexamethasone. In marked contrast, dexamethasone had a significantly (P<0.015) smaller effect on I(sus) (11% increase) in cells from male STZ-diabetic rats, as compared to control cells. However, I(sus) augmentation in cells from female STZ-diabetic rats was normal (31% increase). In ovariectomized-diabetic rats, I(sus) was unchanged by dexamethasone. The reduced effect in diabetic males might be due to preactivation of tyrosine kinases linking dexamethasone to current modulation. 3. In conclusion, type I diabetes is associated with gender-specific changes in sensitivity of K(+) currents to glucocorticoids, linked to alterations in tyrosine-phosphorylated proteins.