Purkinje strands from both ventricles of adult mongrel dogs were excised, and electrical properties were studied by the voltage-clamp technique. The strands were then examined with light and electron microscopy and structural properties were analysed by morphometric techniques. The canine Purkinje strand contains (by volume) about 28% myocyte and 55% dense outer connective tissue. The remainder of the volume is taken up by the inner shell of loosely packed connective tissue within 10 $\mu $m of a myocyte membrane. These volume fractions vary considerably from one strand to another. Clefts less than 10 $\mu $m wide occupy 18% of the myocyte volume and clefts less than 1 $\mu $m wide occupy 1%. The membrane surface area of the myocytes can be divided into three categories by reference to the size of the adjacent cleft. About 47.8% of the membrane surface area faces clefts wider than 1 $\mu $m another 22.2% faces clefts between 0.1 and 1 $\mu $m wide, and the final 30% faces clefts less than 0.1 $\mu $m wide. The surface area facing the narrowest clefts (less than 0.1 $\mu $m wide) is divided between nexuses 3%, desmosomes 10%, and unspecialized membrane 17% (each figure is expressed as a percentage of the total surface area of myocyte membrane). The canine Purkinje strand has a more favourable anatomy than the sheep Purkinje strand for most physiological experiments. We expect that the complicating effects of series resistance and change in the concentration of extracellular ions will be much smaller than in sheep strands, but still not negligible.