Load-independent growth controls were investigated in embryonic atria and ventricles cultured in the anterior eye chamber of adult rats. At embryonic day 12, the mass of atria was half that of ventricles (0.11 vs. 0.24 mg), whereas after 8 wk in oculo, atrial grafts were threefold larger than ventricular grafts (2.26 ± 0.36 vs. 0.69 ± 0.16 mg). To examine possible paracrine communication between atria and ventricles, two grafts were cocultured in the same eye chamber. Atrial grafts cocultured with either another atrial graft or a ventricular graft did not differ in projected area or mass from atrial grafts cultured alone. Ventricular grafts cocultured with another ventricular graft did not differ in projected area or mass from ventricular grafts cultured alone. However, ventricular grafts cocultured with an atrial graft grew larger than single ventricular grafts. At 1 wk in oculo, DNA synthesis (bromodeoxyuridine incorporation) was higher in ventricular grafts cocultured with atrial grafts than in single ventricular grafts (6.84 ± 0.88 vs. 4.40 ± 0.10%). Thus atria may stimulate growth, including DNA synthesis, in developing ventricles via a paracrine mechanism.