In order to find the most suitable animal species for studies on nonimmunologic contact urticaria (NICU), human NICU agents: 20% benzoic acid (BA), 10% sorbic acid (SA), 15% cinnamic acid (CA), 20% cinnamic aldehyde (CAL), 1.0% diethyl fumarate (DEF), 0.2% methyl nicotinate (MN), all in absolute ethyl alcohol, and 100% dimethyl sulfoxide (DMSO) were each applied to the earlobes of guinea pigs, rats, and mice. Guinea pig ear reacted with swelling to all agents tested. The mean maximal increase in thickness of the guinea pig ear, measured with a micrometer, was 114 +/- 46%. No response to BA, SA, CA, DEF, or MN was noted in the rat or mouse ear. However, DMSO and CAL caused significant ear swelling both in the mouse and the rat. Thus, the guinea pig ear is more sensitive to a variety of human NICU agents than is rat or mouse ear. The striking differences in species reactivity to NICU agents suggest the possibility that there are several mechanisms (mediators) involved in reactions to different substances. The guinea pig ear swelling test remains the best quantitative animal method available for screening human NICU agents.