The photoprotein aequorin was injected into cells of ferret papillary muscles to monitor the resting intracellular free Ca concentration [( Ca2+]i). Increasing the external Ca concentration [( Ca2+]o) increased both resting [Ca2+]i and resting tension. The tension and [Ca2+]i both rose to a peak and then declined to a steady-state level which was higher than the control. Qualitatively similar, but larger, effects were observed if [Ca2+]i was first elevated with strophanthidin. The increase of [Ca2+]i was accompanied by the development of spontaneous oscillations of [Ca2+]i. When a steady level of [Ca2+]i had been reached in high [Ca2+]o, [Ca2+]o was reduced back to the control level for a brief period. A subsequent increase of [Ca2+]o produced a rise of [Ca2+]i to the same steady level as that previously found in the high [Ca2+]o but the initial peak and subsequent decline were absent. It is suggested that the decline of [Ca2+]i from the initial peak is mediated by a fall of intracellular Na concentration [( Na+]i) limiting Ca entry on a Na-Ca exchange. Increasing external K concentration [( K+]o) from 5 to 30 mmol/l had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was first increased either by applying strophanthidin or by increasing [Ca2+]o, increasing [K+]o produced a transient rise of [Ca2+]i and tension. This rise was unaffected by D600. It is suggested that the secondary decline of [Ca2+]i after the initial rise may, again, be produced by a fall of [Na+]i acting on an Na-Ca exchange. Acidification produced by increasing [CO2] had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was increased by strophanthidin, acidification produced a rise of [Ca2+]i. This rise of [Ca2+]i was partly transient even when the intracellular acidification was presumably maintained (raising CO2 at constant [HCO3-]). Acidification in Na-free solutions had qualitatively similar effects to those in Na-containing solutions. In Na-free solutions (Na replaced by K) the [Ca2+]i could be maintained at a low level for at least several hours. Increases of [Ca2+]o in Na-free solutions led to a decrease of [Ca2+]i, and similarly decreasing [Ca2+]o led to an increase in [Ca2+]i. These anomalous effects of [Ca2+]o on [Ca2+]i could be abolished by Mn ions or D600. It is suggested that changes in [Ca2+]o may have reciprocal effects on Ca permeability and hence on [Ca2+]i. The application of the mitochondrial uncoupler FCCP in Na-free solutions led to an increase of resting tension followed, after a substantial delay, by an increase of [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)