Nine plant species believed to be important to mule deer (Odocoileus hemionus) on the Cache la Poudre range of Colorado were collected at four seasons during 1961, from five study areas which varied in altitude from 6,400 to 10,300 ft. Eleven chemical components were analyzed for each of the 77 plant samples. In a deciduous species (Purshia tridentata) the percentage dry matter and crude fat, and the caloric content varied from high autumn—winter levels to low spring—summer values; and beta carotene and percentage protein, ash, phosphorus, potassium, calcium, sodium, and magnesium levels varied from low autumn—winter values to high spring—summer values. In an evergreen species (Juniperus scopulorum) the seasonal changes in plant chemistry were not so pronounced. Major variations in the measured components were attributable to two causes in the following order: (1) differences in nutrient levels between species; (2) differences in nutrient levels measured for one plant species at the four seasons. Minor variations were evident within single species when sampled in a specific season at different areas throughout its range. The normal and predictable variation in plant chemistry caused by seasonal changes in plant phenology and physiology and physiology has important implications to the nutrition and physiology of deer.