Utilizing new materials and miniaturization techniques, an ultraminiature catheter pressure transducer for catheterization of the ;ulmonary arter (PA) has been developed and applied in intact, spontaneously breathing, anesthetized rats. The catheter arrangement consists of three components: 1. a SPR-671 ultraminiature pressure transducer (measuring catheter), 2. a plastic introducer (sheath) which is slipped over the measuring catheter, 3. an external wire mounted on the outside of the introducer for bending its tip. First, the measuring catheter is inserted through the right jugular vein into the right ventricle. Then the introducer is slipped over it. The tip of the introducer is bent so that there is an angle of about 90° or less to the shaft. The measuring catheter is advanced across the pulmonary valve into the PA. Measurements of PA pressure were made in 5 male Long Evans (body weight: 364 ± 7 g) and 5 female Sprague-Dawley rats (body weight: 244 ± 7 g) under control conditions. The effect of i.v. infusion of norepinephrine (NE, 0.1 mg/kg/h, 20 minutes duration) was tested in Long Evans rats. PA systolic pressure (PASP) was 34.0 ± 0.8 and 29.5 ± 0.4 mmHg, and PA diastolic pressure (PADP) was 23.6 ± 0.8 and 18.1 ± 0.6 mmHg in male Long Evans and 5 Sprague-Dawley rats, respectively. NE induced an increase of both PASP (40.8 ± 0.1 mmHg) and PADP (28.6 ± 0.4 mmHg) and an elevation in pulmonary vascular resistance from a control value of 0.093 ± 0.003 to 0.103 ± 0.004 mmHgxkgxmin/ml.