This prospective, randomized study examined the effect of baricity of intrathecal preservative-free morphine on the duration of postoperative analgesia and incidence of side-effects in infants and children receiving high spinal anaesthesia with hyperbaric tetracaine in combination with a light general anaesthetic. Fourteen infants and children, aged 7-91 months, undergoing repair of either uncomplicated atrial or ventricular septal defects, were randomized to receive either 10 microg x kg(-1) of intrathecal morphine in combination with 0.5% tetracaine D10 (hyperbaric morphine group) or intrathecal morphine mixed with saline and injected sequentially after the administration of 0.5% tetracaine D10 (hypobaric morphine group). After spinal injection, patients were positioned in 30 degrees of Trendelenburg for a minimum of 10 min. Postoperatively, patients were monitored for a minimum of 12 h. Pain scores and the incidence and severity of side-effects were recorded every 1 h. All patients were extubated at the conclusion of surgery without any incidence of respiratory depression. There was a decreased incidence of vomiting in the hypobaric morphine group and no significant difference in the duration of analgesia. When intrathecal morphine is administered in conjunction with a hyperbaric tetracaine spinal to paediatric cardiac patients in the head down position, sequential administration of the hypobaric solution may mitigate side-effects.