Gal alpha 1,3Gal carbohydrate residues are present in the glycoproteins and glycolipids of lower mammals, and appear to be involved in the binding specificity of several membrane receptors. We report here that endothelial cells stimulated with lipopolysaccharide or inflammatory cytokines modulate their expression of UPD-Gal: beta-D-Gal alpha 1,3-galactosyltransferase (alpha 1,3GT), the Golgi enzyme that attaches a galactose in alpha 1,3 configuration to an N-acetyllactosamine acceptor. Upon activation, the steady state level of mRNA is transiently increased, the modifications being paralleled by a transcriptional regulation of the gene. Cell-associated enzyme activity, on the other hand, falls rapidly after activation, before being up- and downregulated with kinetics that parallel those of the mRNA, and after 3 days reaches a level representing 40-60% of the activity in cells before activation. Overall Gal alpha 1,3Gal expression at the cell surface follows enzyme activity, except that it is insensitive to the rapid and transient reduction of activity occurring shortly after activation. This reduced alpha 1,3GT activity in stimulated EC is correlated with lower stability of the protein, and with a switch in the expression of the isoform pattern, isoform 1 being predominant in resting cells whereas after activation it is isoform 2 that predominates. The two isoforms, however, appear to have similar intrinsic stability, so that the reduced stability of the enzyme in activated EC probably results from an induced proteolytic degradation pathway.