To determine the site of highest resistance to the movement of water across Bruch's membrane in humans. A hydraulic conductivity chamber was designed that enabled us to measure flow across Bruch's membrane while ablating its subepithelial aspect using an excimer laser (193 nm). When resistance was lost, samples were fixed and processed for electron microscopy. Changes in the hydraulic conductivity of Bruch's membrane in response to the excimer-mediated sequential removal of tissue layers was studied in four control eyes of donors 26, 46, 61, and 76 years of age and in one eye of an 83-year-old donor with age-related macular degeneration. The number of laser pulses required to abolish the resistance in Bruch's membrane was found to be dependent on the age of the donor. The ablation rate was approximately 0.11 micron per pulse. Loss of resistance correlated with removal of layers internal to the layer of elastin. This study indicates that the inner collage-nous layer imparts the major resistance to fluid movement between the retinal pigment epithelium and the choroid. Aging changes in the ultrastructure of these compartments could be responsible for the decrease in hydraulic conductivity observed in early life in previous studies.