To investigate the spatial organization of the vagal preganglionic neurons that control gastric acid, insulin and glucagon secretion, as well as bradycardia, these variables were simultaneously monitored before, during, and after monopolar stimulation (50 microA, 1 ms, 50 Hz) through semimicroelectrodes (tip 25-50 microns) in urethan-chloralose-anesthetized rats. Gastric acid, insulin, and glucagon secretion were elicited by stimulation of the dorsal motor nucleus of the vagus (DMV), whereas heart rate, mean arterial blood pressure, and plasma glucose were not affected. Post hoc analyses were used to characterize the viscerotopic patterns within the DMV. For each of the three positive responses, the "center of gravity" (centroid) of 205 stimulation sites was located within the DMV at approximately the midanterioposterior extent of the nucleus. Vectors of response effectiveness around the centroids as well as comparisons of different groups of DMV electrodes established that effective sites were found throughout the full rostrocaudal extent of the DMV. Comparisons of left vs. right DMV sites indicated that gastric acid, insulin, and glucagon secretion are influenced equally by the two sides of the brain stem. The differences in response strength observed in the DMV were best described in terms of the discrete medial and lateral longitudinal columns of the DMV. The centroid point for gastric acid was in the medial DMV, and the points for insulin and glucagon were in the lateral DMV. Further, medial placements were significantly more effective than lateral ones in eliciting gastric acid secretion; lateral placements were more effective than medial ones in eliciting insulin secretion. Although the effect was not significant in the case of glucagon, the trend paralleled the pattern for insulin.