Type III procollagen N-peptides (PIIINPs) are believed to be released in stoichiometric amounts as type III collagen molecules are secreted from cells. We hypothesized that if the human lung actively produces type III collagen a detectable transpulmonary gradient in PIIINPs would exist in normal individuals that might be altered following a pulmonary insult. PIIINPs were therefore measured by radioimmunoassay in serum taken simultaneously from the pulmonary artery (PA) and left ventricle/aorta (LV) in 11 patients undergoing routine cardiac catheterisation. Mean PIIINP levels +/- SEM in LV were 66.8 +/- 5.4 micrograms.ml-1 and 59.9 +/- 4.1 micrograms.ml-1 in PA (p less than 0.04). In 6 patients, repeat measurements taken 4 h after cardiopulmonary bypass revealed a significant fall in PA values to 43.8 +/- 2.6 micrograms.ml-1 (p less than 0.001) and abolition of the transpulmonary gradient. These results suggest the adult human lung actively synthesis type III collagen and that, in the short term, cardiopulmonary bypass inhibits this process.