The effects of endogenous and exogenous glucocorticoids on renal active and prokallikrein levels (ng/mg protein) and in vivo kallikrein synthesis rate were studied in the conscious rat. Within two hours after low dose methylprednisolone (MP, 0.0125 to 0.05 mg/100 g body wt), active kallikrein and prokallikrein fell (29.1 +/- 2.3 and 35.1 +/- 2.7 ng/mg protein, respectively, compared to 38.4 +/- 3.7 and 42.7 +/- 3.4 in vehicle-treated rats, P less than 0.05 or less). These changes were accompanied by a significant fall in prokallikrein synthesis rate relative to total protein synthesis. The reductions in active and prokallikrein levels were transient, dissipating by six hours. With increasing MP doses, there was further dose-dependent reduction in active kallikrein. However, prokallikrein levels increased to normal as the MP dose was increased despite continued suppression of synthesis, suggesting that prokallikrein activation was inhibited. Renal kallikrein levels were also examined in relation to changes in endogenous glucocorticoid levels. In intact rats, three hours after plasma corticosterone peaked (10 p.m.), active and prokallikrein levels were 30.2 +/- 2.9 and 27.0 +/- 1.6 ng/mg protein, respectively, compared to 36.9 +/- 2.3 and 37.2 +/- 2.6 (P less than 0.005) three hours after the corticosterone nadir (11 a.m.). Furthermore, adrenalectomy increased active and prokallikrein (47.3 +/- 4.8 and 87.3 +/- 6.0 ng/mg protein, respectively), compared to levels in intact or shamoperated rats (intact: 32.9 +/- 2.9 and 54.9 +/- 5.3 ng/mg protein, P less than 0.01 or less). Adrenalectomy also eliminated the diurnal changes in kallikrein levels seen in intact rats. These data suggest that renal prokallikrein synthesis and activation are physiologically regulated by glucocorticoids.