If luminal Ca concentrations are high, Ca transport across epithelia occurs mainly via passive, paracellular mechanisms driven by an electrochemical gradient or solvent drag. Restricted supply as well as an increased demand induce an endocrine response, mainly mediated by calcitriol, resulting in a relatively greater contribution of a more efficient, active, transcellular Ca transport. In intestinal and renal epithelia, this is a 3-step process involving apical uptake via Ca channels (TRPV5, TRPV6), protein-bound diffusion through the cell (CaBPD28K, CaBPD9K) and active extrusion at the basolateral membrane (NCX1, PMCA1b). To further improve our strategies to prevent hypocalcemia, applied studies should be combined with fundamental research on the complex regulation of these transport processes, especially because ruminants show some particularities in comparison to monogastric animals. Although the small intestine expresses the same vitamin D-dependent Ca transport proteins (TRPV6, CaBPD9K, PMCA1b), flux rates determined using mucosal preparations from sheep and goats were demonstrated to be much smaller than those detected in samples from pigs or horses. Nevertheless, the expression of the above mentioned transport proteins is regulated and altered by dietary Ca, N restriction and lactation. The rumen has also been shown to actively absorb Ca in many in vitro and in vivo experiments. But the absence of any relevant amounts of TRPV6 or CaBPD9K, the unresponsiveness to dietary Ca restriction or calcitriol treatment and the pivotal role of luminal SCFA indicate an alternative, so far unknown mechanism. Interestingly, lactation and a diet negative in DCAD increased ruminal flux rates. Renal Ca excretion is generally low in ruminants and cannot be significantly diminished to compensate for challenges of Ca homeostasis. But when the animals are kept on a ration negative in DCAD renal Ca resorption is inhibited. Preliminary experiments demonstrated that the expression of TRPV5, CaBPD28K and NCX1 is not significantly altered under these conditions. This might explain why renal resorption is immediately restored when the ration is changed p.p. An adaptation on the functional level occurs faster than the stimulation via the genomic pathway.