A series of related studies were designed to investigate and quantify the degree of fluid exchange between the lung segment and the interstitium or pulmonary circulation that occurred during a standardized bronchoalveolar lavage (BAL). In 5 subjects undergoing a 3 x 60 ml BAL, the dilution of introduced fluid was calculated at approximately 25% using both technetium colloid and methylene blue. Thus, there was a total dilution volume of about 225 ml. In the same experiment, tritiated water was incorporated into the introduced fluid, and the degree of dilution of tritium in the aspirate was compared with that of the other 2 markers. The dilution of tritium was greater than anticipated, suggesting that around 55 ml of water had effluxed from the lung segment during BAL. The total fluid gain by the segment during BAL was thus approximately 100 ml (44% of the dilution volume), although the contribution of fluid resident in the lung prior to BAL to this volume was not known. The 3 x 60 ml BAL procedures were performed in a further 5 patients 12 h after they had received tritiated water orally. The concentration of urea, a putative "endogenous marker" of dilution, was assayed simultaneously in plasma and aspirate. From these values the fluid normally resident in the lung segment was calculated to contribute at most about 2% to the total aspirated volume. A median of 39% of the 85 ml aspirated was calculated to have come from the circulation or surrounding interstitium by simultaneous measurements of the concentrations of tritium in plasma and lavage aspirate.(ABSTRACT TRUNCATED AT 250 WORDS)