ABSTRACT. It has been shown that addition of the surfactant Orvus ES paste (OEP) and its main component sodium lauryl sulfate (SLS) to boar or dog semen before freezing improves post-thaw sperm motility and protects acrosome caps. In this study, we investigated the usefulness of the addition of OEP (0, 1, 2 and 4%) or SLS (0, 1, 2, 3 and 4 mg/ml) to cat ejaculates before freezing and their concentrations. Among the OEP addition groups, the 1% OEP group showed higher sperm motility than the other groups. Among the SLS addition groups, the 3 mg/ml SLS group showed slightly higher sperm motility and viability than the other groups. Comparison between the 1% OEP and 3 mg/ml SLS addition groups suggested a higher percentage of sperm with an acrosome cap in the 1% OEP group. The other sperm properties did not significantly differ between the 2 groups. These results indicate that addition of 1% OEP or 3 mg/ ml SLS is effective for freezing of cat ejaculated semen.KEY WORDS: acrosome cap, cat, frozen semen, orvus es paste, sodium lauryl sulfate.J. Vet. Med. Sci. 72(1): 23-27, 2010 In regard to artificial insemination (AI) with frozen feline semen, intravaginal [6,11,19] and intrauterine [6,18,19] inseminations with ejaculates have each been reported in three studies, and intravaginal [20] and intrauterine [15,20] inseminations with epididymal sperm have each been reported in one and two studies, respectively. In our 2003 year study using epididymal sperm [15], we added 7% (v/v) glycerol and 1% Orvus ES paste (OEP, also known as Equex STM paste, Nova Chemical Sales, Inc., Scituate, MA, U.S.A.) to semen as cryoprotective agents and achieved a conception rate of 27.3%. However, we did not evaluate the usefulness of addition of OEP for cryopreservation of feline semen or its optimal concentration. Addition of OEP along with glycerol for the cryopreservation of boar [12] and dog [16,17] semen has been shown to protect the acrosome caps of sperm, thereby increasing and maintaining post-thaw sperm motility for a long period of time. In 2004, Axnér et al. [5] investigated the usefulness of Equex STM paste for cryopreservation of feline epididymal sperm and showed that addition 0.5% (v/v) Equex STM paste led to greater protection of the acrosome caps of sperm after freeze-thawing than no addition, but sperm motility was lower (sperm survival was shorter) 4-6 hr after thawing than in the case of no addition. Our search of the literature revealed no studies in which addition of OEP for semen cryopreservation adversely affected post-thaw sperm motility. It remains unclear whether the results of the study by Axnér et al. [5] regarding cryopreservation of epididymal sperm apply to cryopreservation of cat ejaculates.OEP is known to contain mainly sodium lauryl sulfate (SLS), but data on its concentration and the remaining constituents have not been published. It has been suggested that the effect of SLS might be exerted by modifying the structure of egg yolk lipoproteins in the extracellular medium [4]. Kato et al. [9] employed ...