The effects of experimental diabetes and insulin treatment on the decreased reactivity of isolated rat duodenum to KCI and calmidazolium, a specific calmodulin antagonist, were examined. After 8 weeks of streptozotocin diabetes, the contractile effect of KCI and the non-competitive antagonistic effect of calmidazolium against KCI on isolated rat duodenum were decreased. Calmodulin levels, as measured by radioimmunoassay, were also found to be decreased in duodenum from streptozotocin-diabetic rats. Neither impaired reactivity to KCI nor decreased calmodulin levels in diabetic rat duodenum were corrected by treatment with insulin (10 IU/kg for 20 days). Following insulin treatment, there was only a partial correction in the antagonistic effect of calmidazolium as shown by the increase in non-competitive antagonist affinity constant.