The water permeability and physical characteristics of the basement membrane (lens capsule) of the crystalline lens of the adult rat have been investigated. The hydraulic conductivity of the basement membrane at low pressure is 50 +/- 9.5 X 10(-12) m s-1 Pa-1 and at high pressure 17 +/- 7.5 X 10(-12) m s-1 Pa-1. This decrease in permeability occurs despite a 75% increase in area of the membrane and a 65% reduction in its thickness. Conventional theories of a membrane possessing pores or of a fibre matrix of filaments of a constant diameter fail to explain the decreasing permeability of the membrane with increasing hydrostatic pressure. The present data suggest that the structure of the membrane is changed by pressure and the coiled filaments of which it is composed are extended as stress is applied to the membrane. If allowance is made both for thinning and for compaction of the membrane and the extension of its area the permeability of the membrane can be predicted satisfactorily at varying pressures. Thus the hydraulic conductivity of basement membrane at a given pressure can be adequately described by the product of a constant and a dimensionless 'deformation coefficient'. This deformation coefficient is equal to the square of the product of the thickness ratio and elongation ratio of the membrane at the given filtration pressure.