Kallikrein has been reported to stimulate callus formation and cell proliferation. It is well-known that vitamin D3 play an role on the metabolism of calcium. However, the relation between vitamin D3 and kallikrein are still poorly understood. We have studied the effect of kallikrein on calcium and vitamin D3 with canine kidney. Mongrel dogs were anesthetized with sodium pentobarbital and prepared by the method of Nakanishi et al. Drugs were infused into renal artery for 60 minutes under three different serum calcium concentrations (Exp. I, Exp. II, Exp. III). Blood samples were collected at the midpoint of the 60 minutes urine collection period. Plasma vitamin D3 concentrations were measured by the metabolites of 25-hydroxyvitamin D3 and 1 alpha,25-dihydroxyvitamin D3. 25-hydroxyvitamin D3 was estimated by competitive protein binding assay and 1 alpha,25-dihydroxyvitamin D3 was determined by radioreceptor assay after separated using HPLC. Electrolytes of sodium, potassium, chloride, calcium and phosphorus in serum and urine, plasma cyclic AMP, glomerular filtration rate, renal blood flow and blood pressure were also measured respectively. Plasma 1 alpha,25-dihydroxyvitamin D3 concentration was found to be decreased by the infusion of kallikrein (0.02 KU/kg/min) in three experimental conditions, especially in Exp. II. Bradykinin (0.02 microgram/kg/min) also caused to kallikrein-like changes of vitamin D3. It is suggested that kallikrein-kinin system is related to inhibit the mechanism of vitamin D3 activation system on kidney.