Expression of Na(+)-dependent transport of phosphate (Pi) was analysed in Xenopus laevis oocytes after injection of poly(A)-rich RNA isolated from the duodenal mucosa of rabbits with increased levels of 1,25-(OH)2-VitD3 (injection of vitamin D3 or low-Pi diet) or from control animals. In parallel, the effect of elevated levels of vitamin D3 was studied in isolated duodenal brush-border membrane vesicles. In brush-border membrane vesicles, the rate of Na(+)-Pi cotransport was found to be doubled after 1,25-(OH)2-VitD3 injections while Na(+)-D-glucose cotransport (measured as a control) was not altered. In X. laevis oocytes, Na(+)-dependent Pi uptake was increased after injection of poly(A)-rich RNA isolated from duodenal mucosa of animals with increased levels of 1,25-(OH)2-VitD3 but not after injection of poly(A)-rich RNA isolated from control animals; between the two groups of mRNA no difference in the expression of the Na(+)-D-glucose transport system was observed. Sucrose density gradient fractionation suggests that mRNA species related to the increased Na(+)-dependent Pi uptake are of average chain lengths between 2 x 10(3) and 3 x 10(3) bases (2-3 kb). It is concluded that in duodenal enterocytes 1,25-(OH)2-VitD3 increases the content of mRNA species of 2-3 kb that might be involved either directly in Na-Pi cotransport or at least in controlling its activity.