Stable strontium balances were determined in man under strictly controlled dietary conditions in control studies and during both oral and intravenous administration of stable strontium. The diet contained about 1 mg strontium per day and most of this (88%) was excreted in stool while 17.5 per cent was excreted in urine, so that the balance was very slightly negative. During the oral intake of 1536 mg strontium per day, given as the lactate, the urinary and the fecal strontium excretions increased markedly and the balance became strongly positive. During the 6-day periods of intravenous infusions of stable strontium as the gluconate, 30-40 per cent of the infused amount was excreted in the urine and 5-10 per cent in stool. A high percentage of the strontium retained during both oral and intravenous administration was excreted in 30 days after the discontinuation of the strontium administration and there was no evidence of long term retention. The administration of stable strontium, given either orally or intravenously, was associated with an increase of the urinary calcium excretion.