To determine the relation between milk intake and plasma levels of beta-casomorphins as a function of age, we studied the levels of beta-casomorphin-7 immunoreactive material (BCMIR) in 2- and 4-wk-old beagle neonates and adult dogs following milk intake. We used three kinds of milk: 1) bovine casein-based milk formula; 2) canine bitch's milk, and 3) soy protein-based (noncasein) formula. Using a new extraction technique, BCMIR was isolated from plasma and assayed by radioimmunoassay at 0, 2, 4, and 6 h after feeding. We found that BCMIR increased significantly in both 2- and 4-wk-old puppies post bovine and canine milk feeding, but not after soy protein formula. Base-line values were about 80-120 fmol/ml and increased to 100-120% 2 h after bovine casein intake in the older puppies and by 4 h in the younger group. BCMIR levels were undetectable in adult dogs before or after bovine milk feeding. Beta-casomorphin-7 added to plasma was rapidly degraded whereas BCMIR was stable. Chromatographic profiles demonstrated that the BCMIR moiety is larger than beta-casomorphin-7. Our data suggest that the appearance of BCMIR in the plasma of neonatal dogs is a result of beta-casein degradation along a permeable gastrointestinal mucosa. We speculate that morphiceptin and beta-casomorphins can be cleaved from BCMIR and may have important physiologic effects.