1. The contribution of the concentrating effect due to shrinkage of the extracellular space (ECS) to cellular K+ efflux on extracellular potassium ([K+]0) accumulation in response to ischaemia was investigated in an isolated, blood-perfused rabbit papillary muscle preparation with a confined extracellular space. 2. The ECS was quantified using either of two extracellular markers, choline or tetramethyl ammonium (TMA), each with specific ion-selective electrodes, as well as by measurement of extracellular resistance (r0). [K+]0 and [Na+]0 were also measured simultaneously using K+-and Na+-selective electrodes. 3. During ischaemia, [K+]0 increased 3-fold from 4.2 ± 0.1 to 12.6 ± 1.0 mM at 10 min (n = 10) analogous to changes in the ischaemic heart in vivo. The ECS decreased to 83.9 ± 3.2% of control measured using 1 mM choline extracellularly (n = 9, P < 0.01) or to 85.7 ± 0.7% of control using 1 mM TMA (n = 6, P < 0.01). Nearly identical decreases in r0 (84.1 ± 2.4%, n = 15, P < 0.01) occurred simultaneously. 4. The small decrease in the ECS contributed only 0.8-0.9 mM to the total increase in [K+]0 of 8.4 mM and had a minor effect on transmembrane K+ flux. No significant differences between the relative changes in [choline] and [Na+]0 were observed. This excluded a major transmembrane Na+ movement during early ischaemia. 5. Bumetanide (10 mM), an inhibitor of K+-Cl- cotransport, a process which is involved in cell volume regulation consequent to osmotic cell swelling, significantly attenuated the increase in [K+]0 after 6 min of ischaemia (8.3 ± 0.6 mM, n = 5 vs. 10.3 ± 0.4 mM in the control group, n = 6, P < 0.05), whereas N-ethylmaleimide (1 mM), a stimulator of this cotransporter, augmented [K+]0 accumulation (12.0 ± 0.6 mM at 6 min, P < 0.05). 6. We conclude that during early myocardial ischaemia, a major component of [K+], accumulation is not caused by diminution of ECS per se, but rather by increased net K+ efflux due in part to K+-Cl- cotransport secondary to myocyte volume regulation.