Proliferating nonmyocardial cells (NMCs) complicate primary heart cultures and may influence myocardial cell (MC) differentiation. In cultures from the day-old rat ventricle, we validated a method to arrest this proliferation; and we quantitated cross-striated cells and the chronotropic response to isoproterenol to assess MC differentiation. MCs were cultured at single cell density using an improved method. By 4 days, all cells could be identified as MCs or NMCs. In cultures treated for 3 days with bromodeoxyuridine (BrdU), 0.1 miw, serial cell counts were unchanged through 11 days. Among 50,000 cells from 110 cultures, 75-80% were MCs. In control cultures without BrdU, NMC density was 3-and 6-fold that in BrdU-treated cultures at days 5 and 8, respectively (P< 0.01), although the MCs were not overgrown. The MCs did not proliferate in either culture. The proportion of living MCs with cross-striations was similar in treated and control cultures at day 5 (72.6% vs. 69.9%, P> 0.05) but was lower in controls at day 8 (86.3% vs. 50.4%, P< 0.01). A sensitive (ED50 10-100 pM), specific chronotropic response to L-isoproterenol was present in both treated and control cultures, but the maximum response was only 20-30% as great in controls on days 4 and 8 (P< 0.01). Baseline beating rates were the same. The MCs had well-differentiated ultrastructure, including a T system. By autoradiography, a maximum 18.4% of MCs had nuclear incorporation of 3H-BrdU at day 8. Media conditioned by NMCs or by the control cultures did not change the cross-striations or isoproterenol response of BrdU-treated cultures. Thus, in a new culture preparation with few and stable NMCs, morphological and functional MC characteristics were different from those of MCs in cultures with proliferating NMCs. We suggest that an MC-NMC interaction can alter MC properties and that this effect should be considered in studies of primary rat heart cultures. The pure, stable, well-differentiated MCs in BrdU-treated cultures will be useful for studying MC growth, repair, and other time-dependent phenomena.