The capacity of the small intestine to actively transport Ca declines markedly with increasing age in the rat. The basal-lateral plasma membrane Ca pump is thought to be an important component of the active transport mechanism. Therefore, the purpose of this study was to determine if there are changes in the expression of the intestinal Ca pump with age, mRNA levels were quantitated by Northern and dot blot analysis using a cDNA probe based on the sequence of the plasma membrane Ca pump expressed in the rat intestine (PMCA1). In the duodenum, Ca pump mRNA levels were 3-4 times higher in young (2 months) rats compared to adult (12 months) and old (27 months) rats. In the ileum, Ca pump mRNA levels were one third those of the duodenum, and ileal levels were higher in young rats compared to adult rats. These changes in mRNA levels with age and segment were significantly correlated with Ca pump activity as measured in basal-lateral membrane vesicles in vitro. To determine intestinal responsiveness to 1,25(OH)2D, rats were fed a strontium diet to induce vitamin D deficiency. In young animals, 1,25(OH)2D significantly increased Ca pump mRNA levels 4-fold in the duodenum. 1,25(OH)2D had a similar effect in the adult duodenum. These studies demonstrate that there are changes in Ca pump mRNA levels with age and intestinal segment. Since there was no change in the capacity of 1,25(OH)2D to increase Ca pump mRNA levels, the decline in Ca pump expression may be due to the age-related decrease in serum 1,25(OH)2D rather than to decrease responsiveness to 1,25(OH)2D.