Means (± SD) of serum electrolytes sampled from 21–32 male and 32–65 female Rocky Mountain mule deer (Odocoileus hemionus hemionus) 18 months and older collected yearlong were: sodium, 148.5 ± 19.0 and 148.8 ± 17.8 meq/liter; potassium, 8.88 ±2.12 and 8.74 ±2.13 meq/liter; calcium, 6.44 ± 1.58 and 6.13 ±1.31 meq/liter; magnesium, 3.11 ± 0.95 and 3.34 ±1.14 meq/liter; and inorganic phosphorus, 7.27 ± 1.03 and 7.82 ± 2.70 mg/100 cc, respectively. Significant (P < 0.05) between-sex differences were limited to higher mean calcium values in 6-11 month old females. Significant age class differences occurred only in mean potassium and inorganic phosphorus among males and mean calcium among females wherein younger deer had the larger values. Significantly larger seasonal means occurred in potassium of mature females during the spring and summer and in inorganic phosphorus of mature females during the summer. Over a three-year period of decreasing precipitation and browse yields, annual means of sodium and calcium increased in mature deer of both sexes but decreased for inorganic phosphorus in mature females.