1. Experiments were performed to examine responses of spinothalamic tract (STT) neurons to distension of the renal pelvis. Nineteen monkeys (Macaca fascicularis) were anesthetized with alpha-chloralose, paralyzed, and artificially ventilated. Fifty-four STT neurons in the T11-L2 segments were studied. Each cell was excited by renal nerve stimulation and had a somatic receptive field in the left flank and/or the abdomen. 2. Distension of the left renal pelvis to 50 mmHg for 20-30 s increased activity of 40 STT neurons. Two types of responses were observed. Six cells responded rapidly to the increase in renal pelvic pressure. Thereafter activity of these cells completely adapted. The other 34 cells also responded rapidly to the distension: however, the subsequent adaptation was not complete. Average activity before distension was 13 +/- 1 (SE) spikes/s. Distension increased activity to a peak of 42 +/- 3 spikes/s. Mean activity just before the end of the distension was 27 +/- 3 spikes/s. 3. The pelvic pressure-cell response relation was determined for 16 cells. Only one cell responded to a pressure of 20 mmHg. Three responded to 30 mmHg, and all others responded to 40 mmHg and higher. The average response threshold was 32 +/- 1 mmHg. Peak responses increased as distending pressure increased from 40-80 mmHg. Responses to a pressure of 100 mmHg were no greater than to 80 mmHg. Adapted levels of activity were also a function of distending pressure in the 40-80 mmHg range. 4. Probability of responses was unrelated to somatic input. However, cells with A delta- and C-fiber renal input were significantly more likely to respond to renal pelvic distension than cells with only A delta-renal input. Magnitude of responses to a pressure of 50 mmHg was not related to the type of renal input to the cells; however, among the cells tested at all pressures, cells with A delta- and C-fiber input had significantly greater responses to pressures of 80 and 100 mmHg. 5. Cells were studied in laminae I and IV-VII: responses were unrelated to laminar location. None of the 6 cells located in L2 responded to renal pelvic distension; 8 of 12 in L1 responded; 24 of 28 in T12 responded; and all 8 cells in T11 responded. 6. Stimulation of inhibitory receptive fields on the right hindlimb reduced activity of four cells to a significantly greater extent during pelvic distension than before pelvic distension.(ABSTRACT TRUNCATED AT 400 WORDS)