With the purpose of searching for new clinical features in labile hypertension (LH), systolic time intervals were recorded non-invasively in a group of LH patients. Obtained data were compared with a matched group of normal individuals and also with a group of young patients with chronic essential hypertension. An increase in heart rate and a reduction in carotid pulse upstroke time (Ut) in labile hypertensives differentiated this group from chronic hypertensives and normal subjects. A tendency towards a reduction in heart rate, together with an increase in Ut and ejection time during a two years follow-up study, was accompanied by stable recordings of high blood pressure values in most of the patients belonging to the LH groups. Considering LH as the initial, reversible stage of a pathologic process, changes in heart rate, Ut and ejection time, together with high blood pressure values reflect an evolution to chronicity.