Hematocrit (Hct) changes during mental stress were studied in 21 patients with mild hypertension. A 10 min arithmetic stress test increased blood pressure from 129 +/- 2/80 +/- 2 mmHg (mean +/- SEM) to 167 +/- 5/98 +/- 3 mmHg, and heart rate from 64 +/- 2 to 81 +/- 3 (p less than 0.001). Concomitantly, a significant (p less than 0.001) increase in Hct was observed (from 41.5 +/- 0.7% to 42.2 +/- 0.8%), and increments of Hct (-0.1% to 2.2%) were significantly (r = 0.61, p less than 0.01) correlated with those of systolic blood pressure. Plasma levels of norepinephrine (NE), epinephrine (E) and atrial natriuretic peptide (ANP) also showed a significant increase after mental stress. All these changes disappeared after a 60 min recovery period. The results suggest that mental stress increases Hct promptly, which may be due to concomitant increases in plasma NE, E and ANP levels. Hct elevation resulting from mental stress may be clinically relevant, at least, in hypertensive patients who are at risk of occlusive vascular diseases.