Changes in blood pressure, heart rate and the time necessary for recovery from motor block after spinal anesthesia were compared in 65 children, aged 8 months to 15 years, and 17 adult patients. All patients had similar levels of sensory anesthesia (T5-T3) following tetracaine, 0.3 mg/kg, plus phenylephrine, 0.075 mg/kg. Children less than 5 years of age showed little or no change in blood pressure and heart rate following spinal anesthesia, but children more than 6 years old had widely variable decreases in blood pressure. The decreases of blood pressure in the older children resembled those in adults. The time needed to regain motor function increase with age; in children less than 2 years of age, motor function returned in about a fifth of the time needed in adults. It is suggested that 1) the small proportions of the lower extremities and immaturity of the sympathetic nervous system in young children are responsible for the cardiovascular stability observed in these children during spinal anesthesia; 2) age-related differences in duration of motor block reflect physical and physiologic differences, including amount of cerebrospinal fluid, diameter and surface area of the spinal cord and nerve roots, and rate of absorption of local anesthetics from the subarachnoid space.