OBJECTIVE To explore sources of serum aldosterone concentration variability in a population of healthy and diseased ferrets, determine a preliminary 1 -sided reference interval for serum aldosterone concentration in healthy ferrets, and identify a decision limit to differentiate healthy from diseased ferrets on the basis of serum aldosterone concentration. DESIGN Prospective threshold definition and diagnostic accuracy study. ANIMALS 78 healthy (n = 56) and diseased (22) ferrets. PROCEDURES Serum aldosterone concentrations were measured on consecutively admitted ferrets, and an upper reference limit for aldosterone concentrations was established. Sensitivity and specificity of aldosterone concentration cutoffs to differentiate healthy from diseased ferrets were estimated with receiver operating characteristic curve analysis. RESULTS Measurements of serum aldosterone concentrations in the ferrets showed wide variability, with a median concentration of 4.75 pg/mL (interquartile range, 0.55 to 17.9 pg/mL; range, 0.02 to 283.9 pg/mL) and 76% (59/78) of samples having concentrations < 18 pg/mL. Ferrets that were healthy, older, or sexually inactive had significantly lower aldosterone concentrations. The upper limit of the reference interval for healthy ferrets was 13.3 pg/mL (90% confidence interval, 9.9 to 16.9 pg/mL). Analysis of receiver operating characteristic curves indicated that an aldosterone concentration cutoff value of 7.6 pg/mL differentiated healthy ferrets from diseased ferrets with a sensitivity of 72.7% and specificity of 73.2% (area under the curve, 0.79; 95% confidence interval, 0.67 to 0.91). CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that high aldosterone concentrations should not be considered diagnostic of primary hyperaldosteronism in ferrets. A need exists to develop better tests to identify primary hyperaldosteronism.