The calcium-sensitive photoprotein aequorin was micro-injected into cells of rat, ferret, rabbit and cat papillary muscles. Aequorin light emission is a function of free intracellular calcium concentration. The changes in intracellular calcium concentration [( Ca2+]i) and tension accompanying changes of pH have been studied. When the solution perfusing the papillary muscle was changed from Tyrode solution equilibrated with 5% CO2 to Tyrode solution equilibrated with 15% CO2, developed tension showed a rapid fall followed by a slower rise to a steady state which was less than the control. However the calcium transient associated with each contraction increased monophasically to a new steady state. When the external pH was held constant during exposure to 15% CO2 (by increasing the [HCO3-]), the initial fall of tension was reduced and the slow recovery of tension was greater than when CO2 alone was changed. The amplitude of the calcium transient increased monophasically to a new steady state which was greater than control, but less than when [CO2] alone was increased. If [HCO3-] was decreased while maintaining [CO2] at 5%, there was a slow monophasic decline in developed tension, and a small increase in peak light. Alkaloses produced by changing the [HCO3-]/[CO2] ratio produced similar results but the changes observed were in the opposite direction to those described above. The effects of changes of pHo can be explained if pHi affects tension by two mechanisms. The first mechanism, which is responsible for the rapid change in tension, is not associated with a change in [Ca2+]i. The second mechanism leads to a slower and smaller change in tension, in the opposite direction to the first, and is due to a change in the intracellular calcium transient.