Heat production by resting smooth muscle, was measured with a heat-flux micro calorimeter on cut-open segments of pig coronary artery superfused at 30 °C, was 0.93 ± 0.06 (n = 16) mWg wet weight. Time courses of the increases in isometric tension and heat production with respect to basal during sustained supra maximal acetylcholine stimulation were qualitatively similar: initial peak tapering down to a supra basal plateau. Mean tension-associated heat production over 1 h was 0.16 Jg. During sustained exposure to phorbol 12, 13-dibutyrate, supra basal tension and - with a 5–10 min initial delay - supra basal heat progressively increased to a plateau in about 40 min. Mean tension-associated heat production over 1 h was only 0.02 Jg with normal extracellular and intracellular mobilizable Ca₂ pools, and it was further reduced to 0.01 Jg with depleted Ca₂ pools. These results show that the maintenance — if not necessarily the building up - of tension under phorbol 12, 13-dibutyrate does not entail any large dissipation of energy and is not dependent on the presence of normal Ca²⁺ pools.