Alarm reaction and serum K+ in hypertensive patients. The effect of serum K+ of the alarm reaction induced by the participation to an experimental noninvasive study was evaluated in 35 subjects with borderline hypertension and in 18 essential hypertensives. A group of 44 inpatients undergoing routine blood sampling served as a control. Serum K+, blood pressure and heart rate were measured before (casual) and after (baseline) 20 min of rest in the recumbent position. Baseline serum K+ values were significantly higher than casual values in patients participating to the experimental protocol while no change was observed in inpatients undergoing routine blood sampling. The increase in serum K+ induced by relaxation was significantly related to heart rate decrease (r = 0.73). After relaxation 75% of patients had an increase in serum K+ with a change greater than 10% in about 35% of patients. In a subgroup of patients who repeated the same test three times, the alarm reaction was still evident and not reproducible within each patient. These data suggest that when potassium levels are measured in outpatients undergoing diagnostic or experimental procedures falsely reduced levels can be found in a large proportion of subjects. 