The in vitro stimulation of the accumulation of p-aminohippurate (PAH) was investigated in renal cortical slices of 10- and 55-day-old rats. For this goal tissue slices were incubated for 24 hours in Williams medium E containing different concentrations of dexamethasone (0.01-5 micromol). After this incubation period slices were placed in PAH containing Cross-Taggart medium and PAH uptake into the kidney tissue was measured for two hours. In both immature and adult rat's tissue slices an active PAH accumulation can be proven after 24 hours incubation without hormone substitution. In both age groups the accumulation capacity after 24 hours incubation reaches about 40-60% of that of acute two hours incubation experiments (slice to medium ratios after two hours of incubation: immature rats: 9.5+/-0.4; adults: 11.4+/-0.3). In the acute incubation experiments there was no influence of dexamethasone on PAH accumulation whereas after 24 hours incubation dexamethasone significantly increases the PAH uptake capacity in a concentration dependent manner: 10- and 55-day-old rats: 26 and 36 (65) % increase, respectively. It can be concluded that 1.) a stimulation of tubular transport capacity can be performed in vitro, and 2.) discrepancies between in vitro and in vivo stimulation exist concerning PAH accumulation in renal cortical slices.