The effects of changing external Ca concentration ([Ca]o) on contractile parameters and heat production were investigated in the interventricular rabbit septa and the dog papillary muscle. Double reciprocal plots of tension development as a function of [Ca]o yielded half-maximal activation values of 1.04 +/- 0.17 and 2.8 +/- 0.7 mM Ca for the septum and papillary muscle, respectively. Resting heat rate was similar in both preparations, 1.9 +/- 0.08 mW . g-1 for the septum and 1.7 +/- 0.07 mW . g-1 for the papillary muscle, and it was not altered by changes in [Ca]o. Active heat production (Ha) normalized per unit of force developed (19 +/- 1.3 microJ . mN-1 . g-1) for the septum and the dimensionless ratio Ha/(To . lo), (0.30 +/- 0.02) for the papillary muscle, where To is the isometric tension and lo, the muscle length, remained unaltered with changes in [Ca]o. Total heat production per beat normalized per unit of force developed (Ht/T) for the septum and the ratio Ht/(To . lo) for the papillary muscle decreased hyperbolically with [Ca]o. Therefore, as a result of the unaltered economy of the contractile system and the unchanged resting heat rate, muscle economy improves as [Ca]o approaches physiological levels. Further increase in [Ca]o, over the physiological levels, can only slightly improve muscle economy.