Prostaglandin E₂ (PGE₂) level in rectal mucosa excised from 17 patients suffering from ulcerative colitis was 2-fold higher than that found in rectal mucosa of 17 normal subjects: 2.0 ± 0.4 and 0.9 ± 0.2 ng per mg of wet tissue, respectively. Accumulation of PGE₂ in 24-hr cultures of rectal mucosa specimens obtained from patients with ulcerative colitis was 112% higher than that observed in cultures from control subjects. Addition of sulfasalazine, sulfapyridine, and 5-aminosalicylic acid to the culture medium of ulcerative colitis mucosa resulted in inhibition of PGE₂ production by 34, 32, and 62%, respectively, compared to rectal specimens cultured in drug-free medium. These results suggest that PGE may mediate the inflammatory response in ulcerative colitis and that some of the therapeutic effect of sulfasalazine and its constituents are related to the inhibition of PGE synthesis.