Labile hypertension in patients under 50 years of age (the non-atherosclerotic form) was found to be characterized by higher urinary excretion of catecholamines and particularly of homovanillic acid; when further analyzed it was shown to be a heterogeneous entity with two types of patients clinically and biochemically distinguishable from each other, from control subjects and from patients with stable hypertension. Reactivity to assuming an upright posture distinguishes one type of labile hypertension having a normal postural pulse rate response from another having an excessive postural increase in pulse rate. The first group also showed normal responses of plasma norepinephrine concentration and of urinary cyclic AMP to posture. The group with excessive pulse rate response, in contrast, showed a decrease in plasma norepinephrine and an excessive increase of urinary cyclic AMP excretion in response to upright posture. The results suggest that not only circulating catecholamines but also the reactivity of their target tissues (as probably reflected by cyclic AMP measurements) are important in bringing about signs of adrenergic excess. The hypothesis that cyclic AMP changes reflect beta-adrenergic receptor reactivity is strongly favoured by data indicating qualitative differences in cyclic AMP responses to beta-adrenergic stimulation or inhibition between control subjects and those labile hypertensive patients with clinical signs of excessive sympathetic reactivity. The study stresses the need for more precise definition of labile hypertension, for dynamic clinical and biochemical correlative studies, and for consideration not only of the circulating hormones but also of the "second messengers" (such as cyclic AMP and cyclic GMP) which reflect the cellular action of hormones. Blood pressure is a very labile parameter in health and disease. In one sense, therefore, hypertension can be considered "labile" in every hypertensive patient. Usually, however, labile (or borderline) hypertension is regarded as characterized by a blood pressure over 140/90 mmHg, falling below these values with physical and emotional rest. This clinical entity, which affects some 20% (variously estimated between 16 and 30%) of the adult population, gives rise to uncertainties in both definition and prognosis. In some patients labile hypertension represents the precursor of a fixed hypertensive state, whereas in many others it remains labile throughout life, never progressing to the stable phase nor becoming associated with hypertensive cardiovascular disease.(ABSTRACT TRUNCATED AT 400 WORDS)