The availability of specific cDNA probes to the chick intestinal calbindin-D28K (CaBP) mRNA has allowed us to assess the regulation of this mRNA in response to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration. It has previously been demonstrated that dietary calcium and phosphorus can effect alterations in the steady-state intestinal levels of chick CaBP. We have examined whether or not perturbations in dietary calcium and phosphorus have an effect on the expression of the intestinal mRNA coding for CaBP in the vitamin D-replete chick. We found altered protein levels of CaBP as expected; however there was surprisingly no difference in steady-state CaBP mRNA levels between the different dietary groups. These data suggest that calcium and phosphorus regulation of CaBP occurs at a post-transcriptional level. In addition, we have examined what effect dietary manipulations of calcium and phosphorus levels have on the response of the vitamin D-replete intestine to 1,25(OH)2D3 administration as assessed by CaBP mRNA changes. Administration of 1,25(OH)2D3 to vitamin D-replete chicks maintained on normal calcium and phosphorus levels resulted in a less than 2-fold increase in CaBP mRNA levels. Previous studies have demonstrated that receptor occupancy goes up 6-fold under these conditions; therefore there is apparently a very tight regulation of CaBP gene activity. 1,25(OH)2D3 administration to chicks raised on either low calcium, high calcium, or low phosphorus vitamin D-replete diets similarly showed only small changes in the intestinal CaBP mRNA levels; however there seemed to be qualitative differences in response attributable to the dietary alterations.