The rapid (1–4 h) responses of epithelial target tissues to mineralocorticoids contrast with the days/weeks apparently required for responses in the cardiovascular system. The present study explores the time course and pattern of early events leading to cardiac fibrosis in the mineralocorticoid-salt rat model. Uninephrectomized rats were given deoxycorticosterone (20 mg, sc, weekly) plus 0.9% NaCl/0.3% KCl to drink and were killed at 2, 4, 8, 16, and 32 d. Type III collagen increased progressively from d 2, and blood pressure from d 4, with 4 and 8 d rats showing marked perivascular inflammatory cell infiltration. Apoptosis was also noted in perivascular areas at 4 and 8 d and in scar areas at 8, 16, and 32 d. Elevation of mineralocorticoid hormone levels inappropriate for salt status thus provokes a series of changes in cardiac vessels and myocytes leading to increased collagen deposition. When mineralocorticoid levels are elevated acutely by bolus injection, changes are discernible after 2 d, in contrast with previous infusion studies in which 3–4 wk were required for measurable changes.