Catecholamine release from chromaffin cells in the adrenal medulla is a vital element of the stress response. The activity of large conductance Ca2+-and-voltage-gated potassium channels (BK channels) is an important regulator of chromaffm cell excitability. The subunits, ?2, ?4, and the stress-regulated Slo-? splice variant, STREX, modulate the kinetic properties of BK channels and their sensitivity to chemicals. Previous research has found gender dimorphisms in the expression of Slo-?, ?2, and STREX mRNA in adrenal medullae from adult SJL mice, with males showing significantly higher expression than females. Recent literature has also identified chronic-stress-dependent increases in the expression of Slo and STREX mRNA in the adult male SJL adrenal medulla. The SJL mouse strain is of interest, as adult SJL males differ from females in size, behavior, and adrenal morphology. This investigation used quantitative real-time RT-PCR to examine changes in the expression of Slo, STREX, ?2, ?3, and ?4 mRNA in the SJL mouse adrenal medulla with respect to gender, stress, and age. Among adult mice tested for Slo, STREX, and ?2, males showed significantly higher mRNA expression in all subunits compared to females. Male adolescent mice showed higher mRNA expression of Slo, STREX, ?2, ?3, and ?4 compared to females, significantly for ?2. Stress increased the expression of all BK subunits in adolescent mice, significantly for ?2, ?3 and STREX. In age comparisons, adolescent mice showed significantly higher expression of ?2 mRNA and significantly lower expression of Slo and STREX mRNA compared to adults. These results suggest male mice have more excitable chromaffin cells than do females and provide evidence for a role of chronic stress in shaping excitability and, in turn, levels of catecholamines in the blood.