To distinguish among genetic, neurohumoral, and hemodynamic explanations for structural and functional differences in the hearts of young spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) control rats, embryonic SHR and WKY rat heart tissue was cultured in the anterior eye chamber of adult SHR and WKY rats. In study 1, atria from E-12 WKY rat embryos grafted into anterior eye chambers of either SHR or WKY host rats achieved a larger size than did SHR grafts by 8 weeks in oculo (2.98±0.75 and 2.55±0.32 mm2 vs. 1.80±0.20 and 2.04±0.44 mm2). Beating rates did not differ between SHR and WKY rat atria implanted into SHR or WKY host rats. In study 2, ventricles from E-13 embryonic SHR and WKY rat hearts grew to similar size and weight when implanted into SHR or WKY host rats (e.g., SHR hearts, 1.81±0.32 vs. 1.74±0.33 mm2; WKY rat hearts, 1.75±0.29 vs. 2.29±0.32 mm2). Ventricle grafts from SHR embryos into SHR host rats beat more rapidly (165±19 beats/min) during weekly measurements than either WKY rat ventricles (92±9 beats/min in SHR host and 99±9 beats/min in WKY host rats) or SHR ventricles grafted into WKY host rats (109±7 beats/min, p<0.001). In study 3, atria from E-13 SHR and WKY rat embryos were grafted into sympathectomized and intact eye chambers of SHR or WKY host rats. Sympathectomy of the eye chamber compromised growth of grafts into WKY host rats (1.54±0.24 vs. 0.90±0.14 mm2) but not SHR hosts (1.54±0.25 vs. 1.73±0.24 mm2). Grafts into sympathectomized eye chambers of WKY host rats beat more slowly than grafts into eye chambers with sympathetic innervation intact (282±14 vs. 202±14 beats/min); sympathectomy did not alter beating rate of grafts in SHR hosts (266±14 vs. 255±18 beats/min). These results suggest that the growth and beating rate of SHR atrial grafts may be less sensitive to sympathetic innervation than WKY rat atrial grafts. In these studies, SHR grafts did not grow larger than WKY heart grafts and did not show an increased intrinsic beating rate, suggesting that the cardiac hypertrophy and increased intrinsic beating rate observed in intact SHR are unlikely to result from direct genetic programming.