The effect of intestinal anaphylaxis on goblet cell mucus release was tested in rats immunized with small doses of egg albumin and alum and challenged intraduodenally with antigen. The alteration in vascular and mucosal permeability which accompanies intestinal anaphylaxis was reflected by increased retention of 125I-labelled rat serum albumin in gut wall segments and increased amounts of protein-bound radioactivity in the intestinal secretion from the segments. Intestinal goblet cell mucus was labelled in vivo with 35S. Infusion of antigen, into the duodenum of actively immunized rats led to the appearance of 35S-labelled high molecular weight glycoprotein, presumably of goblet cell origin, in the intestinal secretions. Goblet cell mucus release was dependent on the dose of antigen infused, was antigen-specific and was inhibited by pretreatment of rats with cyproheptidine. Enhanced release of goblet cell mucus was observed in normal rats prepared by intravenous injection of rat antiserum rich in IgE anti-egg albumin antibodies and challenged by intraduodenal infusion of antigen. Prior heating of the antiserum inhibited passive transfer of the reaction; this finding is consistent with the heat lability of IgE antibodies. The latter class of antibodies are presumed to be responsible for intestinal anaphylaxis and its associated mucus release in the model system examined.