The effects of halothane on SR Ca2+ regulation were studied in saponin-permeabilized rat atrial myocytes in the presence of normal and reduced levels of ATP. Rats (250-300 g) were killed humanely (Schedule 1) and atrial myocytes isolated by enzymatic digestion. Cells were perfused with weakly Ca2+-buffered solutions approximating to the intracellular milieu and Ca2+ release from the SR was detected using Fluo 3. The solution contained (mM): KCl, 100; HEPES, 25; EGTA, 0.36; phosphocreatine 10; ATP, 5-0.05 and Fluo-3, 0.002. The free [Ca2+] and [Mg2+] was 200 nM and 1 mM respectively. pH 7.0, 22 oC. In the presence of 0.36 mM EGTA, spontaneous Ca2+ sparks were readily detected, but propagated Ca2+ release did not occur. Exposure to 1 mM halothane for 2-minutes had no effect on the amplitude or frequency Ca2+ sparks in the presence of 5 mM ATP. The solution was then changed to one of similar composition, but lacking ATP. Consistent with previous findings (Yang & Steele, 2001), this resulted in a decrease in the frequency of spontaneous Ca2+ sparks and an increase in amplitude. After 2-minutes exposure to the zero ATP solution, the Ca2+ spark frequency decreased from 3.4 ± 0.3 to 1.1 ± 0.15 events per second (mean ± s.e.m., n=53). On subsequent introduction of halothane a sudden burst of large amplitude Ca2+ sparks occurred and spark frequency transiently increased to 4.05 ± 0.6 events per second (n=53). The amplitude and frequency of halothane-induced Ca2+ sparks then declined progressively over 1-2 minutes. Further experiments were carried out to assess the effects of halothane on the SR Ca2+ content under these conditions. Brief application of caffeine (20 mM) was used to release Ca2+ from the SR and the resulting fluorescence transient was used as an index of the SR Ca2+ content. Halothane (1 mM) had no effect on the SR Ca2+ content in the presence of 5 mM ATP. Decreasing [ATP] from 5 mM to 0.05 mM resulted in a progressive increase in the SR Ca2+ content over 1-2 minutes. However, no increase in SR Ca2+ content occurred when 1 mM halothane was present during ATP depletion. The ATP-dependence of these effects suggests that halothane may have little direct effect on SR Ca2+ regulation under normal physiological conditions. However, when cells become metabolically impaired, halothane may serve to maintain the sensitivity of the RyR as the [ATP] decreases, thereby limiting the rise in SR Ca2+ content that would otherwise occur. This may reduce the severity of spontaneous Ca2+ release during ischaemia and subsequent reperfusion.