The purpose of this study was to compare, in the same subjects, hormonal responses to 30-min head-up tilt (HUT) and lower body suction (LBNP) of different intensity (24° and 70°, and 15 and 35 mm Hg, respectively). Basal pooled individual data from -10 min (n=32) were within normal reference limits: norepinephrine (NE) averaged 318±23 pg/ml; epinephrine, 34.0±5J> pg/ml; plasma renin activity (PRA), 0.72±0.08 ng ATII/ml/h; aldosterone, 164±20 pg/ml; atrial natriuretic peptide (ANP), 29.9±2.0 pg/ml; cGMP, 6.29±0.59 mmol/1; cortisol, 95.7±5.8 ng/ml; and ACTH, 50.3±2.6 pg/ml. The low-level stimuli failed to induce consistent changes in hormone levels. From the onset of the stimulus (minute 0) to its termination (minute 30), norepinephrine (NE) increased by 101 % with LBNP-35, and by 70 % with HUT70, respectively. The NE increase with LBNP-35 was higher (p<0.05) than with HUT70. Epinephrine rose with HUT70 (by 162 %) only. PRA increased by 157 % with LBNP-35, and by 119 % with HUT70, respectively; these responses were not significantly different. Aldosterone rose equally (by 85 and 89 %) with LBNP-35 and HUT70 but not with the low-level stimuli. No consistent changes were observed in ANP, c-GMP or ACTH concentrations. Cortisol values fell during the LBNP and HUT24 situations but rose transiently after HUT70. We conclude that the hormones investigated respond differently to head-up posture and lower body suction and in a specific manner. Greater effects of high-level stimuli (HUT70, LBNP-35) were noted as compared to low-level stimuli (HUT24, LBNP-15). The application of combined sets of models stimulating the cardiovascular system may aid in the analysis of responses of hormonal systems in man.