Thirty-six former preterm infants undergoing inguinal hernia repair were studied. All were less than or equal to 51 weeks postconceptual age at the time of operation. Patients were randomly assigned to receive general or spinal anesthesia. Group 1 patients received general inhalational anesthesia with neuromuscular blockade. Group 2 patients received spinal anesthesia using 1% tetracaine 0.4-0.6 mg/kg in conjunction with an equal volume of 10% dextrose and 0.02 ml epinephrine 1:1000. In the first part of the study, infants randomized to receive spinal anesthesia also received sedation with im ketamine 1-2 mg/kg prior to placement of the spinal anesthetic (group 2 A). The remainder of group 2 patients did not receive sedation (group 2 B). Respiratory pattern and heart rate were monitored using an impedance pneumograph for at least 12 h postoperatively. Tracings were analyzed for evidence of apnea, periodic breathing and/or bradycardia by a pulmonologist unaware of the anesthetic technique utilized. None of the patients who received spinal anesthesia without ketamine sedation developed postoperative bradycardia, prolonged apnea, or periodic breathing. Eight of nine infants (89%) who received spinal anesthesia and adjunct intraoperative sedation with ketamine developed prolonged apnea with bradycardia. Two of the eight infants had no prior history of apnea. Five of the 16 patients (31%) who received general anesthesia developed prolonged apnea with bradycardia. Two of these five infants had no prior history of apnea. When infants with no prior history of apnea were analyzed separately, there was no statistically significant increased incidence of apnea in children receiving general versus spinal anesthesia with or without ketamine sedation. Because of the small numbers of patients studied, and the multiple factors that may influence the incidence of postoperative apnea (e.g., prior history of neonatal apnea), standard postoperative respiratory monitoring of these high-risk infants is still recommended following all anesthetic techniques.