A novel technique is described for the assay of acute gastric irritancy of non-steroidal anti-inflammatory drugs (NSAIDs) in mice in which (a) the gastric mucosa is sensitized to the irritant actions of the drugs by co-administration of bethanechol chloride to increase acid and pepsin production, and (b) the area and number of haemorrhagic lesions in the glandular mucosa is measured quantitatively by visual image analysis. The technique has been used to assess the acute gastric irritancy of 20 NSAIDs in mice. In relation to published values for their acute and chronic anti-inflammatory activities, drugs with low relative gastric irritancy (e.g. carprofen, chloroquine, diclofenac, fenbufen, tenoxicam, tilomisole) were differentiated from the drugs of higher relative irritancy.