The development and use of electrophysiological methods in the study of the permeability of single capillaries are described. On the whole, the values obtained with single vessels correspond satisfactorily to those obtained from whole organs by use of tracer techniques. The findings support the contention that whole organ techniques allow assessment of capillary permeability with a reasonable accuracy. There is a large range of small solute permeabilities among different 'continuous' capillaries spanning from 7 x 10(-4) cm sec-1 in the mesentery to 3 x 10(-7) cm sec-1 in the brain. The finding of different permeabilities between capillaries in various organs can be rationalised in terms of the pore theory. Thus the results comply with a physical membrane having pore densities ranging from 10 per micron2 (mesentery) to 0.01 per micron2 (brain). Since-in all likelihood-ion permeation primarily takes place via the junctional zones between endothelial cells these differences probably refer to subtle differences in the nature and complexity of junctional contacts.