Absorption of $^{45}Ca^{2+}$ was studied in the small and large intestine of the adult male frog Rana pipiens through an in situ intestinal ligated-loop preparation. With a graded series of luminal calcium concentrations, $^{45}Ca$ uptake into small intestine in March animals is by passive diffusion alone, while in April through June $^{45}Ca$ uptake in duodenum and jejunoileum follows a curvilinear function that shows a saturable and a nonsaturable component. The duodenal nonsaturable component had a diffusion constant ($K_{d}$) of 0.71 ±.09 mL/g/hr. The saturable component followed Michaelis-Menten kinetics, with a saturable flux ($J_{max}$) of 23.9 ± 2.5 μmol/g/h and half-saturation constant ($K_{m}$) of 9.5 ± 2.1 mM Ca²⁺. Jejunoileum also exhibited nonsaturable and saturable components with a $K_{d}$ of 0.65 ± 0.09 mL/g/h. Colon exhibited only a passive diffusion uptake pattern with a $K_{d}$ of 0.40 ± 0.03 mL/g/h, which is significantly lower than that of the small intestine. After 500 μg of vitamin ...