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The effect of the light–darkness cycle on the efficiency of the ultimobranchial and parathyroid glands in altering duodenal calcium transport and plasma and urinary concentrations of calcium was examined in the adult male frog (Rana pipiens). Frogs were unfed, but were allowed access to 0·05 M-CaCl₂ in the surrounding medium after ultimobranchialectomy or parathyroidectomy. Calcium transport, as assayed by the everted gut-sac technique, was increased in ultimobranchialectomized frogs at sunrise, concomitant with acute hypercalcaemia and hypercalciuria. An opposite but chronic response was observed in parathyroidectomized frogs with intact ultimobranchial glands. The maximum response observed at sunrise occurred when the concentration of calcium in the plasma of control frogs was decreasing; the minimum response, which occurred 6 h after sunrise, was coincident with a diurnal peak in the concentration of calcium. Vitamin D₃ (500 µg/frog) enhanced calcium transport in ultimobranchialectomized frogs, which resulted in chronic hypercalcaemia and hypercalciuria. The results suggest that diurnal variations in the plasma concentration of calcium do not initiate ultimobranchial activity, but are a response to endocrine control synchronized with the transition from darkness to light.

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