Fos expression induced by nociceptive mechanical distention of the proximal colon was examined in the lumbosacral spinal cord in freely moving rats equipped with a chronic balloon in the proximal colon. Fos protein in lumbosacral neurons was detected immunocytochemically, and colocalization with nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) activity was determined histochemically at 1 hour after distention. Distention of the proximal colon (10 ml, 30 seconds on/off for 10 minutes, about 90 mm Hg) increased the number of Fos-positive cells in the lumbar 6 (L6) and sacral 1 and 2 (S1, S2) segments, whereas no change was observed in the L1-L5 and S3 segments compared with the sham distended group or with animals that received no treatment. In L6-S2 segments, Fos-positive neurons were increased by two-fold in laminae I-VII (mainly in laminae I and outer II) and area X (surrounding the central canal) and by nine-fold in the sacral parasympathetic nucleus. Results of time course studies indicate that the maximal increase in Fos expression observed at 1 hour after distention returns to basal levels within 4 hours. In the S1 segment, distention of the proximal colon increased the percentage of NADPHd/Fos-positive neurons selectively in the parasympathetic nucleus by 40% compared with less than 4% in the sham distention group; the number and pattern of NADPHd-stained cells were not modified. These results indicate that noxious distention of the proximal colon for a short duration in awake rats selectively activates neurons in the L6-S2 segments of the dorsal horn mainly in laminae involved in nociceptive and autonomic processing. The marked activation of NADPHd-positive neurons in the sacral parasympathetic nucleus suggests a possible role of nitric oxide in the visceroautonomic reflexes induced by distention of the proximal colon.