The effect of 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3) administration on Mg metabolism in sheep was studied. Three wethers were intramuscularly injected with 0.2 microgram/kg body weight of 1 alpha-OH-D3 for 3 days. Plasma Mg concentration was decreased and plasma Ca concentration was increased by 1 alpha-OH-D3 administration. However, the duration of hypomagnesemia was shorter than that of the increment in the plasma Ca concentration. The administration of 1 alpha-OH-D3 increased apparent Ca absorption and urinary Ca excretion. However, apparent absorption and urinary excretion of Mg was not changed by 1 alpha-OH-D3 administration. The amount of glomerular filtrated Mg was reduced concomitantly with the reduction of plasma Mg concentration and glomerular filtration rate when 1 alpha-OH-D3 was injected. The tubular reabsorption of Mg tended to be suppressed and the ratio of reabsorbed to filtrated Mg was significantly lowered by 1 alpha-OH-D3 administration. These results suggested that the stimulation of urinary Mg excretion might not be one of the major factors inducing hypomagnesemia although 1 alpha-OH-D3 suppressed reabsorption of Mg in the renal tubule.