1. Vitamin D and its metabolites were measured in the plasma of five species of rural domestic animals. 2. Concentration of 1,25-dihydroxyvitamin D was higher (P less than 0.01) in young animals (range 24-118 pg/ml, means +/- SD = 72.0 +/- 30.0) than in adult animals (range 14-67 pg/ml, means +/- SD = 40.2 +/- 22.6). 3. 25-Hydroxyvitamin D3-26,23 lactone was present only in the chick and the pig. 4. Unsheared sheep appeared to be inefficient utilizers of the photochemical conversion of 7-dehydrocholesterol to vitamin D3. 5. Conversion of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D was most efficient in species with high plasma phosphorus concentrations (pig, sheep).