A myooid is a three-dimensional skeletal muscle construct cultured from mammalian myoblasts and fibroblasts. The purpose was to compare over several weeks in culture the morphology, excitability, and contractility of myooids developed from neonatal and adult rat cells. The hypotheses tested were as follows: (1) baseline forces of myooids correlate with the cross-sectional area (CSA) of the myooids composed of fibroblasts, and (2) peak isometric tetanic forces normalized by total CSA (specific Po) of neonatal and adult rat myooids are not different. Electrical field stimulation was used to measure the excitability and peak tetanic forces. The proportion of the CSA composed of fibroblasts was greater for neonatal (40%) than adult (17%) myooids. For all myooids the baseline passive force normalized by fibroblast CSA (mean = 5.5 kPa) correlated with the fibroblast CSA (r2 = 0.74). A two-element cylindrical model was analyzed to determine the contributions of fibroblasts and myotubes to the baseline force. At each measurement period, the specific Po of the adult myooids was greater than that of the neonatal myooids. The specific Po of the adult myooids was ~1% of the control value for adult muscles and did not change with time in culture, while that of neonatal myooids increased.