The effect of pharmaceutical excipients on the gastrointestinal mucosa in rats was examined by measuring the permeation of phenosulfonphthalein (phenol red), a poorly absorbed drug. The urinary recovery after oral administration of phenol red was significantly increased by pretreatment with sodium lauryl sulfate and Triton X-100 in rats. However, pretreatment with polysorbate 80, lactose, fructose, mannitol, calcium citrate, acacia, or polyethylene glycol 4000 had no apparent effect.