1. The photoprotein aequorin was micro-injected into papillary muscles from the right ventricle of ferrets and rats. Tension and aequorin light (a function of intracellular [Ca2+]) were monitored. 2. In stimulated ferret papillary muscles, increasing the [CO2] of the bicarbonate-buffered superfusate from 5% (pH 7.35) to 20% (pH 6.8) led to a rapid decrease of developed tension, with no significant change in the size of the intracellular Ca2+ transient which accompanies contraction. There was then a small brief recovery of tension which was accompanied by a large brief increase in the size of the Ca2+ transient. Tension then declined again before recovering more slowly, with no significant change in the size of the Ca2+ transient. 3. The time course of the Ca2+ transient was prolonged on exposure to the acid solution, but shortened on continued exposure to the acid solution. Relaxation of twitch tension became faster on exposure to the acid solution, but slowed again on continued exposure to the acid solution. 4. In the presence of 10 mM-caffeine the size of the Ca2+ transient increased during the initial decline of developed tension, the short-lived recovery of tension was abolished, and the Ca2+ transient became smaller during the slower recovery of developed tension. 2 microM-ryanodine had similar effects on developed tension. 5. Addition of 10 mM-lactic acid to the superfusate produced changes similar to those described in 2 and 3 above. 6. An intracellular acidosis, produced by the addition and subsequent withdrawal of 20 mM-NH4Cl from the superfusate also caused changes similar to those described above. In the presence of caffeine, withdrawal of NH4Cl produced changes similar to those described in 4 above. 7. In unstimulated ferret papillary muscles, increasing superfusate [CO2] produced an increase of aequorin light when the bathing [Ca2+] was increased or in the presence of ouabain (10 microM). This increase was not inhibited by verapamil (5 microM), carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM) and oligomycin (2.5 microM), but was reduced by ryanodine (2 microM). 8. Rat papillary muscles showed responses which were quantitatively different from those observed in ferret papillary muscles: the initial recovery of tension developed more slowly, and sarcoplasmic reticulum (s.r.) inhibitors had a greater inhibitory effect on the recovery of tension. 9. It is concluded that the early decline of developed tension observed during acidosis is due to a decrease in Ca2+ release by the s.r. and a decrease in Ca2+ binding by the myofilaments.(ABSTRACT TRUNCATED AT 400 WORDS)