Urinary excretion of antimony, arsenic, bromine, cadmium, caesium, cobalt, copper, gold, iron, mercury, molybdenum, ribidium, scandium, selenium, silver, tungsten and zinc from five hypertensive patients beforeand during treatment with hydralazine has been investigated. The method consisted of neutron activation analysis combined with a recently developed ion exchangee technique. The mean copper excretion during treatment with hydralazine was found to be more than twice that before treatment.