To allow experimental manipulation of adrenal hormone and autonomic influences on developing myocardium without alteration of hemodynamic load, embryonic rat heart was cultured in the anterior eye chamber of an adult rat. Sympathetic innervation of embryonic day 12 heart grafts was manipulated by surgical sympathectomy of one eye chamber in each host rat. Adrenal hormone exposure was manipulated by host adrenal medullectomy (MEDX) in experiment 1 and by host adrenalectomy (ADX) in experiment 2. In experiment 1, whole heart grafts were larger in MEDX than in sham-operated hosts by 8 wk in oculo (6.14 ± 0.71 vs. 5.09 ± 0.69 mm2 with innervation intact and 7.97 ± 2.07 vs. 3.09 ± 0.63 mm2 with sympathetic innervation prevented). In experiment 2, host ADX increased growth of embryonic day 12 ventricles grafted into sympathectomized eye chambers (0.69 ± 0.10 vs. 0.44 ± 0.04 mm2) but did not affect growth of grafts in intact eye chambers (0.85 ± 0.09 vs. 1.05 ± 0.15 mm2). Corticosterone replacement (4 mg/day) entirely reversed the effect of host ADX on graft growth (superior cervical ganglionectomy, 0.47 ± 0.03 mm2; intact eye chambers, 0.90 ± 0.91 mm2). Beating rate of grafts was not affected by adrenal hormone manipulations. These experiments indicate that the compromised growth of embryonic heart grafts placed in sympathectomized eye chambers requires exposure to adult levels of glucocorticoids during the early days after grafting. These results suggest that interactions between neural and hormonal stimulation influence cardiac growth in the in oculo culture system and during normal development.