SummaryWe investigated with anin vivosingle-pass perfusion technique net secretion of calcium, magnesium, and zinc from proximal and distal small intestinal segments and from the cecum plus colon segments of suckling, weanling, and adolescent rats during perfusion with either isotonic (300 mOsm/kg) or hypertonic (500 mOsm/kg) mineral-free solutions. There was net secretion of calcium, magnesium, and zinc in all segments perfused at all age periods.During isotonic perfusion, rates of net secretion of calcium and zinc were severalfold greater in the proximal and distal segments of the suckling rats compared with the corresponding segments of the adolescent rats. Net magnesium secretion was similar in the suckling and adolescent rats. Net secretion rates for the weanling rats tended to be intermediate. During hypertonic perfusion, net secretion rates for calcium, magnesium, and zinc were severalfold greater from all segments of the suckling rats compared with the corresponding segments of the adolescent rats. During either isotonic or hypertonic perfusions, rates of net secretion of calcium and magnesium in general were greater from the proximal and distal segments than from the cecum plus colon in the suckling and weanling rats. In contrast, for the adolescent rats, net secretions from all segments in general were similar. For zinc, rates of net secretion were somewhat greater in the cecum plus colon of rats in all age groups.Our findings suggest a greater permeability of the intestinal epithelium, not only to water and electrolytes but also to certain minerals (calcium and magnesium) and trace elements (Zn) in suckling rats compared with adolescent rats. The implication is that during periods of osmotic diarrhea, infant animals appear to be at risk for losing excessive amounts of minerals and trace elements. These findings may have clinical relevance to human infants suffering from recurrent diarrheal diseases.