Platelet-activating factor (PAF) has been implicated in the pathogenesis of several inflammatory pulmonary diseases, and specific binding sites have been demonstrated in human and guinea pig lung membranes by radioligand binding experiments. Both human and guinea pig PAF receptors (PAFR) have recently been cloned. We have used molecular probes to study the gene expression of PAFR in human and animal lung and in situ hybridization to determine the distribution of PAFR mRNA in peripheral lung. Northern blot analysis of total lung RNA from human lung parenchyma, using a 1.1-kb SmaI-EcoRI fragment of human PAFR cDNA or a 0.9-kb SmaI-SmaI fragment of guinea pig PAFR cDNA, demonstrated the expression of PAFR mRNA in human lung, with a single transcript of 4 kb. There was a significant increase in PAFR mRNA in the lungs of asthmatic patients and a significant decrease in PAFR mRNA in the lungs of cigarette smokers compared with normal patients. Similarly, the expression of PAFR mRNA on guinea pig and rat lung was detected as a single transcript of 3 kb, using both guinea pig and human PAFR cDNA probes. A full-length 1.8-kb human leukocyte PAFR cDNA probe was used as the DNA template for producing 35S-labeled antisense and sense cRNA probes for use in in situ hybridization studies of human peripheral lung. These studies revealed high levels of PAFR mRNA hybridization in blood vessels, moderate levels of hybridization in alveolar walls and peripheral airway smooth muscle, but no specific signal in airway epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)