Following treatment of rats of various age groups with calcitriol (2 micrograms/kg b.w., 3 days, once daily) renal tubular transport of p-aminohippurate (PAH) is reduced in kidney cortical slices prepared from 5-, 15-, 33-, and 55-day-old rats. The decrease in renal excretion of PAH following such treatment has a longer degree and is demonstrable in 15- and 33-day-old rats only. Repeated treatment with calcitriol provokes a decrease in body weight, statistically significant in 5- and 55-day-old rats. In 33- and 55-day-old rats repeated treatment with calcitriol (2 micrograms/kg b.w., 3 days, once daily) causes an increase of kidney weight. Repeated treatment with calcitriol is connected with an increase in plasma level and in renal excretion of calcium, statistically significant in most age groups. In rats of various age groups, renal tubular transport of PAH can be stimulated by repeated administration of triiodothyronine (T3; 100 micrograms/kg b.w., 3 days, once daily). This increase in PAH transport, measured in kidney cortical slices, cannot be observed in rats simultaneously treated with calcitriol and T3. Obviously, the expression of higher transport rates for PAH can be prevented by renal effects of calcitriol. Simultaneous treatment with T3 and calcitriol causes an increase in kidney weight and a decrease in body weight in rats of various age groups.