From venous tracer-dilution curves recorded after 36 pulse injections of ⁴²KCl and ¹³¹I-labeled albumin into the coronary artery inflow of 15 isolated canine heart preparations, we calculated maximal fractional extractions (E_max) and capillary permeability-surface area products (PS_cap) for ⁴²K⁺ over a range of plasma flows (F_P) from 0.3 to 1.7 ml min⁻¹ g⁻¹. At low F_P (<1.0), E_max was 0.60 ± 0.06 (mean ± SD) and PS_capwas 0.72 ± 0.20 ml min⁻¹ g⁻¹; at high F_P (>1.0), E_max decreased to 0.49 ± 0.05 and PS_cap increased to 1.06 ± 0.18. Continuous recording (gamma detector) of residual myocardial ⁴²K⁺ in seven hearts showed that the mean fractional escape rate of tracer between 30 and 60 min after injection was 0.011–0.023 min⁻¹; higher rates were observed at high F_P, when the residue of ⁴²K⁺ decreased to less than 10 % of the injected dose by 60 min. Using PS_cap measured at high F_P and considering the virtual intracellular volume of distribution for K⁺ to be 20 ml/g, we calculated the permeability-surface area product for sarcolemma (PS_cw) as 0.54–0.73 ml min⁻¹ g⁻¹, or about 50 % of PS_cap. Considering sarcolemmal surface area (S_cw) as 4,200 cm²/g and capillary surface area (S_cap) as 500 cm²/g, cell permeability is low, with P_cw: P_cap being less than 0.08.