Chemical sympathectomy significantly decreased the contraction force of the papillary muscle in 20- and 40-day-old rats in the right ventricle and in all age categories in the left ventricle (Fig. 3). CF of the intact rats increased with age in both ventricles and it was 32.5±2.8 a.u., 53.2±4.1 a.u. and 58.5±5.5 a.u. at stimulation frequency of 1 Hz in the right ventricle at the age of 20, 40 and 60 days, respectively (Fig. 3, the left panel.