Use of nonpesticidal chemicals to minimize or eliminate horn fly (Haematobia irritans) populations on cattle could be helpful in combating insecticide resistance. One recent approach is the use of natural products with repellent properties against the target pest. A rapid on-animal evaluation system that accounts for both host and pest interactions and bypasses the need for costly multiyear field assessments is needed. The objective of this study was to develop a system to quantify product repellency of horn flies on cattle in a laboratory setting. Animal pair treatment groups were utilized to assess product efficacy and carrier effects across three experimental trials. A treatment solution consisting of 3% geraniol in mineral oil was the natural product repellent. Horn fly populations on animal pairs receiving the geraniol treatment were significantly reduced when compared with untreated animal pairs. However, animal pairs receiving a mineral oil-only treatment showed reduced horn fly populations similar to animals treated with mineral oil plus geraniol. Thus, there was a strong carrier effect that could be difficult to distinguish in the field but was easily observed using laboratory methods.