Peritoneal dialysis provides a convenient means of administering calcitriol. We investigated in vitro the efficiency with which this approach would deliver the drug to patients. We used an injectable preparation of calcitriol, Calcijex (Abbott Laboratories, Montreal, Quebec, Canada), which we radiolabelled by adding radioactive 1,25-dihydroxyvitamin D3 to it. The preparation was injected into dialysis bags and drained through appropriate tubing to monitor delivery of the radiolabelled calcitriol. By 2 h after injecting calcitriol into dialysis bags, 50% of the dose was left in the fluid, by 20 h only 26% was left. The delivered drug was pure 1,25-dihydroxyvitamin D3 based on chromatographic analysis of the recovered radiochemical. To study what would happen if the drug was in the bag for a minimal length of time the calcitriol was injected immediately prior to draining the fluid. We recovered 62.9% +/- 5.2% SD of the dose from the tubing which would theoretically have entered a patient. There was no significant change in calcitriol dose delivered if the fluid was warmed to 37 degrees C prior to injecting the drug and immediate drainage of the bag.