Investigations in genetic forms of experimental hypertensions revealed certain haemodynamic, metabolic and humoral abnormalities in experimental animals already during the prehypertensive period. With regard to the obvious ratio of hereditary factors in the pathogenesis of human essential hypertension (EH), the objective of the present study was to test whether also in healthy normotensive subjects with a positive family history of EH some metabolic and humoral deviations can be detected, as compared with offspring from normotensive families. The authors compared therefore selected biochemical and humoral parameters in 20 sons of hypertensive parents (SH) with the findings in 20 sons from normotensive pa families (SN). SH had, as compared with SN, a significantly higher systolic BP (119 +/- 2.59 > 111.0 +/- +/- 2.04 mmHg). The trend of higher basal blood sugar levels 5.03 +/- 0.15 > 4.70 +/- 0.41 mmol/l) and the higher concentration of immunoreactive insulin (81.4 +/- 9.54 > 70.4 +/- after a glucose load +/- 7.78 microU/l) did not reach statistical significance. In SH plasma concentrations of adrenaline, noradrenaline and dopamine were significantly higher as well as the atrial natriuretic factor (11.7 +/- 0.77 > 8.4 +/- 0.40 fmol/ml) and of endothelin (18.2 +/- 1.70 > 12.7 +/- 0.87 fmol/ml). A load of 75 g glucose raised, as expected, the blood sugar level, IRI and C-peptide, but reduced unexpectedly the endothelin concentration in both groups. As to other biochemical parameters (fibrinogen, sodium, potassium, urea, creatinine, uric acid, cholesterol, HDL- and LDL-fractions, triacylglycerols), no significant differences were found between SH and SN. The finding of a raised mass of the left ventricle and certain differences in the diastolic and systolic left ventricular function are discussed in another paper. The results indicate that in young men with a positive family-history of EH already certain haemodynamic, metabolic and humoral deviations exist before clinical manifestation of hypertension which could contribute to later development of EH and its organ complications.