In immature rats, renal excretion of p-aminohippurate (PAH) can be increased by daily pretreatment with triiodothyronine (T3) or tetraiodothyronine (T4) beginning on the 2nd day after birth (10 micrograms/100 g b.wt. i.p.). The increase of PAH excretion is nearly of the same extent, if 5-, 10-, 20-, 30-, 50-, and 105-day-old rats were pretreated with thyroid hormones (10 micrograms/100 g b.wt. i.p., 3 days, once daily). There is no strongly dose dependent renal effect of T3 and T4, respectively. The time course of stimulation of renal PAH excretion was also characterized in rats of different ages. Simultaneous pretreatment of young and adult rats with cyclopenthiazide and T3 is not distinctly more effective as both components given alone. Particularly the low degree of stimulation in young rats receiving cyclopenthiazide cannot be pronounced by additional T3 administration. Inhibitors of protein biosynthesis (azauracil, neomycin) can antagonize the stimulation of renal p-amino-hippurate excretion in rats receiving T3. The presented data indicate that T3 is effective in young rats. Furthermore, T4 can be converted to T3 in young rats, too. The importance of an intact protein synthesis seems to be a prerequisite to stimulate the kidney function by T3 pretreatment.