The aim of this study was to assess the effects of L-arginine in primary pulmonary hypertension (PPH). Diagnostic cardiac catheterization was performed in 4 patients (pts) (1 man and 3 women, aged 18-47 years) with suspected PPH. In all of them diagnosis of PPH was confirmed; mean pulmonary artery pressure (PAP) ranged from 46 to 83 mmHg. Then 61/min oxygen was administered for 10 min through the oxygen mask (first oxygen test). After another 15 min, L-arginine was infused into an antecubital vein at a dose of 12.63g of L-arginine hydrochloride in 300 ml of 0.9% NaCl over 90 min. 15 min before the planned termination of the infusion the second oxygen test was performed in the same way as the first one. Hemodynamic data were collected by means of two catheters placed in the main pulmonary artery and in the aortic root. Cardiac output (CO) was estimated by the thermodilution technique. Blood samples were drawn from both catheters to estimate oxygen tension and cyclic GMP (cGMP) levels. In pts 1 and 2 differences between baseline values and following L-arginine did not exceed 9% for mean PAP (mPAP), total pulmonary resistance (TPR), mean aortic pressure (mAP), systemic resistance (SR), CO and HR. In patient 3 mAP and SR dropped by about 30%. In patient 4 after 15 min of the infusion mAP and SR fell by about 50%, whereupon we stopped L-arginine administration. Thus, for ethical reasons, we decided not to recruit new subjects for the study. In pts 1-3 aortic oxygen tension diminished by 10-15% on L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)