We tested the hypothesis that in left ventricular myocardial hypertrophy (LVH) the positive staircase effect is impaired and is related to a raised intracellular [Na⁺] ([Na⁺]_i). Human myocardial specimens were obtained from patients undergoing mitral and aortic valve surgery, the latter group had LVH. LVH was induced in guinea-pigs by ascending aortic constriction. The extent of hypertrophy was quantified by measuring myocyte cross-section area, echocardiographic mass (in humans) and heart-to-body weight ratio (in guinea-pigs). The response to increasing stimulation frequency was expressed as the ratio of tension generated at 1.6 and 0.8 Hz (T _1.6/0.8); ratios greater and less than 1.0 equate with positive and negative force/frequency relationships respectively. [Na⁺]_i was measured using ion-selective microelectrodes. In human and guinea-pig myocardium T _1.6/0.8 values decreased and [Na⁺]_i increased with hypertrophy. For guinea-pig myocardium T _1.6/0.8 decreased from 1.39±0.05 to 1.02±0.05 and [Na⁺]_i increased from 7.3±1.4 to 12.1±1.3 mM in LVH. There was a close relationship between the reduction of T _1.6/0.8 and increase of [Na⁺]_i which was also observed when the [Na⁺]_i was increased with strophanthidin in normal myocardium. The recovery of a raised [Na⁺]_i after an acute acidosis was slowed in hypertrophied myocardium and stabilised at a higher level, suggesting that the membrane mechanisms that regulate [Na⁺]_i are reset.