The mechanism of tubular uptake of labeled human growth hormone ([125I]hGH), a low molecular weight protein (approximate 21,5000 daltons), was studied in isolated perfused rat kidneys. Fractional reabsorption (FR) of [125I]hGH was decreased from 94 to 77% over a period of 80 min as perfusate oncotic pressure was lowered by reducing the albumin concentration from 7.5 to 2.5 g/100ml, whereas greater reductions in fractional sodium (delta 35%) and fluid reabsorption (delta 42%) occurred, indicating that tubular [125I]hGH uptake is likely a specific process not directly dependent upon net fluid and sodium reabsorption. Absolute absorption rates of [125I]hGH filtered loads were inhibited by cytochalasin B, a microfilament disrupter when kidneys were perfused with either albumin concentration. Cytochalasin B inhibited [125I]hGH absorption in both a dose-and time-related manner. The low dose of cytochalasin B (2.5 micrograms/ml) decreased [125I]hGH absorption without significantly altering sodium, fluid or glucose reabsorption. With high doses (5 and 10 micrograms/ml), cytochalasin B affected tubular absorption of [125I]hGH to an extent much greater than sodium, fluid and glucose reabsorption. Inhibition of cytochalasin B on FR[125I]hGH was poorly correlated with the concurrent inhibition of FRNa and FRH2O. Accordingly, tubular reabsorption of [125I]hGH is not directly linked to that of sodium, fluid and glucose. The present studies are consistent with the hypothesis that renal absorption of low molecular-weight proteins is via an endocytotic process involving microfilaments.