['We investigated the actions of isoflurane and propofol on neuronal windup in the spinal cord of intact rats. We hypothesized that propofol would depress windup more than isoflurane.', 'In a cross-over design, rats received 0.8 and 1.2 minimum alveolar concentration (MAC) isoflurane and 0.8 and 1.2 ED50 (effective dose(50%)) of propofol, as recordings were made from single units in the lumbar cord (n = 13). Electrical stimuli were applied (20 stimuli at 0.1, 1, and 3 Hz). Neuronal responses were analyzed for those occurring in the C-fiber range (100-400 ms after each stimulus), combined C-fiber and afterdischarge range (100-1000 ms) and the 100-333 ms range for the 3 Hz stimuli. Absolute windup was also calculated (the sum of action potentials for 20 stimuli - 20 x response to the first stimulus).', 'At 1 Hz, total action potentials (mean, standard error) summed across the 20 stimuli (100-1000 ms range) were 571 +/- 106 and 742 +/- 214 for isoflurane (at 0.8 and 1.2 MAC) and 586 +/- 148 and 641 +/- 143 for propofol (at 0.8 and 1.2 ED50), respectively (P = NS); corresponding values for the 0.1 Hz stimuli were 345 +/- 104, 370 +/- 108, 430 +/- 86, and 403 +/- 106 (P = NS), and for the 3 Hz stimuli (100-333 ms range) were 266 +/- 66, 333 +/- 76, 343 +/- 85, and 252 +/- 72 (P = NS). Absolute windup in the 100-1000 ms range was greater for 1.2 MAC isoflurane at 1 Hz (445 +/- 82, P < 0.01), when compared with absolute windup at 0.8 MAC isoflurane and 0.8 and 1.2 ED50 propofol (232 +/- 31, 88 +/- 65, and 210 +/- 41, respectively).'] Do isoflurane and propofol have similar effects on spinal neuronal windup at concentrations that block movement?