In a small number of studies and isolated case reports, intrathecally administered clonidine has been reported to relieve intractable cancer pain and to prolong spinal anesthesia induced by various local anesthetics. A double-blind placebo-controlled clinical trial was carried out in order to evaluate the effect of intrathecal clonidine on pain following cesarean section. Twenty patients who underwent elective cesarean section received, 45 min after general anesthesia, either 150 micrograms (n = 10) clonidine or saline (control group, n = 10) intrathecally. Pain scores were lower in clonidine- than saline-treated patients from 20 to 120 min after intrathecal injection, as measured by a visual pain linear analog scale (P less than 0.05). Pain relief, in terms of the first supplemental analgesic request by patients, lasted 414 +/- 128 min after intrathecal clonidine and 181 +/- 169 min (mean +/- SD) (P less than 0.01) after saline. Clonidine decreased systolic, diastolic, and mean arterial pressures compared to baseline values (P less than 0.05), but heart rate and central venous pressure were unaffected (difference not significant). Maximal reduction of systolic arterial pressure was 15 +/- 9%, of diastolic arterial pressure 22 +/- 12%, and of mean arterial pressure 18 +/- 12%. Clonidine did not affect arterial hemoglobin oxygen saturation or PaCO2. Patients in the clonidine group were significantly more sedated (P less than 0.05) and more frequently reported a dry mouth (P less than 0.01) compared to the normal saline group.(ABSTRACT TRUNCATED AT 250 WORDS)