Objective : We sought to determine whether angiotensin II (Ang II) promotes hypertrophy of cardiac myocytes directly or via paracrine mechanisms mediated by cardiac fibroblasts. Methods: We studied neonatal rat cardiac myocytes and fibroblasts in culture as a model system. Paracrine effects of Ang II were identified using conditioned medium and co-culture experiments. Results: Ang II type 1 (AT₁) receptors responsible for myocyte growth localized to fibroblasts in radioligand binding, emulsion autoradiography, Western analysis, and immunofluorescence staining experiments. The bulk of AT₁ receptor binding in myocyte cultures (1343±472 sites/cell) was to Ang II receptors on contaminating fibroblasts (9747±2126 sites/cell). Ang II induced significant paracrine trophic effects on myocytes in conditioned medium (40% increase in protein synthesis over control) and co-culture (4-fold increase over control) experiments. TGF-β₁ and endothelin-1 were paracrine mediators of hypertrophy in neutralization experiments. Conclusions: Ang II stimulates cardiac myocyte hypertrophy via paracrine release of TGF-β₁ and endothelin-1 from cardiac fibroblasts in a neonatal rat cell culture model.