Pigs demonstrate an increased sensitivity and susceptibility to Escherichia coli heat-stable enterotoxin (STa) in the 1st wk of life and immediately after weaning. To determine the possible mechanisms for this increased susceptibility, we compared STa binding, guanylate cyclase activation, and photoaffinity cross-linking to porcine jejunal brush border membranes prepared from immature (≤1 wk of age)versus adult pigs as well as 3-wk-old weaned versusunweaned pigs. The STa binding capacity of immature pigs was nearly twice that of adult pigs (11.73 ± 1.52 versus 6.00 ± 0.96 x 10-11 mol/L, p < 0.001), and the STa binding capacity of weaned pigs was nearly three times greater than that of unweaned pigs (17.48 ± 2.10 versus 4.86 ± 1.02 x 10-11 mol/L, p < 0.001). Scatchard analysis suggested a single class of STa receptor, with an association of binding constant of ~109 L/mol at all ages. Maximum guanylate cyclase response (expressed as pmol cyclic GMP generated/mg brush border membrane protein/min) was greater in immature versus adult pigs (1312 ± 831 versus 320 ±92, p<0.02). Weaned pigs had a greater maximum guanylate cyclase activation than unweaned pigs (1126 ± 692 versus 624 ± 298); however, this difference was not statistically significant. Autoradiograms demonstrated specific cross-linking of l25I-STa to a number of distinct radiolabeled bands (62, 66, 84, 92, 160, and 165 kD). There was a difference in the size and trypsin sensitivity of these radiolabeled bands as a function of age and weaning. Treatment with trypsin decreased the intensity of the 160 to 165-kD bands while increasing the intensity of the 62- to 66- and 84- to 92-kD bands. These differences in STa binding, guanylate cyclase activation, and STa receptor size may increase the susceptibility of pigs during the 1st wk of life and at weaning to STa-mediated diarrheal disease. © 1991 International Pediatric Research Foundation, Inc.