Nine aryl 4-guanidinobenzoates were synthesized as inhibitors of the sperm enzyme acrosin. These esters were prepared from 4-guanidinobenzoic acid and a number of phenols which had been approved by the FDA for clinical use. The vaginal contraceptive activity of the inhibitors was evaluated in the rabbit at nonspermicidal concentrations (0.1 mg/ml). All the inhibitors except the 2'-carboxamidophenyl and the 2'-isopropyl-5'-methylphenyl 4-guanidinobenzoates caused significant reductions in fertilization compared to the controls. Several of the aryl 4-guanidinobenzoates appeared to be particularly effective. Nonoxynol-9, under the same conditions but at 10- and 100-fold higher concentrations, also showed an antifertility effect. However, even at these increased dose levels, the contraceptive efficacy of nonoxynol-9 was no higher than that of most of the inhibitors and was less consistent than that of the most active aryl 4-guanidinobenzoates. The relatively high in vivo antifertility activity exhibited by several of the aryl 4-guanidinobenzoates encourages their further evaluation as vaginal contraceptive agents.