The authors report their experience with low doses (0.007-0.015 mg/kg), moderate doses (0.016-0.025 mg/kg), and high doses (0.026-0.035 mg/kg) of intrathecal morphine for postoperative analgesia after selective dorsal root rhizotomy surgery in 50 children, aged 3 to 12 years. After closure of the dura, a single dose of preservative-free morphine was injected into the subarachnoid space, and patients were assessed for 48 hours for level of comfort and side effects. The three doses of morphine provided equivalent analgesia and similar side effects. The duration of postoperative analgesia ranged from 3 to 48 hours (mean, 12.2 +/- 9.5 h). Common side effects were limited to nausea and vomiting (42%) and mild facial pruritus. No patient experienced late respiratory depression or generalized pruritus. The authors conclude that low doses of intrathecal morphine is as effective as moderate or high doses of morphine for reducing pain in the immediate postoperative period. Intrathecal morphine provides excellent analgesia after selective dorsal rhizotomy.