1 Treatment of eight healthy males with propranolol (80 mg twice daily) for 6 weeks resulted in a significant reduction in overnight plasma levels of prolactin and LH. 2 Plasma testosterone levels were elevated whilst GH and cortisol were unchanged by such treatment. 3 Measurement of overnight hormone levels 48 h after discontinuing treatment showed no evidence of a ‘rebound’ phenomenon. 4 Cortisol, GH, prolactin, and testosterone plasma levels all showed time dependent changes: propranolol treatment significantly altered the time course of cortisol but not of the other hormones. 5 The effects of chronic propranolol treatment are discussed in terms of a probable direct central action of the drug. In addition the lowered plasma prolactin levels may directly contribute to the hypotensive action of propranolol.

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Propranolol withdrawal is characterized by a potentially dangerous rebound 'increase in adrenergic activity, which is exaggerated in the presence of hyperthyroidism (Ross, Lewis & Henderson, 1979, Lancet, i, 875). Hyperthyroidism itself results in features suggesting increased adrenergic activity. We have investigated the possibility that changes in thyroid hormone activity could contribute to the propranolol-withdrawal syndrome.