A novel cardiac catheterization technique was devised to investigate the PVR in intact spontaneously breathing rats (ISBR) under physiologic conditions with constant left atrial pressure and controlled lung blood flow maintained within normal ranges. The pressure-flow curves in lungs in vivo were curvilinear, were reproducible for as long as 3 hours, and were not altered by 1-1.5 hrs of arrested perfusion, cyclooxygenase blockade, or changing perfusate from aortic blood to mixed venous blood. In contrast to in vitro isolated perfused rat lung studies, L-NAME reversed the PVR to both acetylcholine and bradykinin and increased baseline pulmonary arterial pressure suggesting EDRF modulates pulmonary arterial pressure at physiologic flows. Unlike previous results obtained from in vitro perfused rat lung studies, the present results in ISBR demonstrate the PVR to adrenomedullin (ADM) and CGRP were not altered by specific ADM and CGRP receptor antagonists, respectively. Since marked differences exist between the pulmonary vascular bed of the ISBR in the present study and the pulmonary vascular bed of the in vitro isolated perfused rat lung in earlier studies, the present data suggest that such differences critically influence the relevance of the in vitro isolated perfused rat lung model.