2-B-P-041 〇森 大輝¹、満倉 靖恵²、沼田 洋輔¹、林田 健一郎¹ Daiki Mori¹, Yasue Mitsukura², Yosuke Numata¹, Kenichiro Hayashida^{1 1}新日本科学・薬効薬理研究部、²慶應義塾大・理工・システムデザイン工学科 ¹Department of Pharmacology. Shin Nippon Biomedical Laboratories, Ltd., ²Department of System Design Engineering, Faculty of Science and Technology, Keio University Cynomolgus monkeys, like humans, are continuous breeders and while the female sexual cycle has been extensively studied, little is known about male sexual cycles, such as periodic changes in the testicle size. Although testosterone, the primary male hormone secreted by the testicles, has been demonstrated to increase pain thresholds in humans and animals, whether testicle size affects pain thresholds is unknown. In the present study, we measured the testicle size in 7 matured male cynomolgus monkeys (6-8 years old) once a week for 23 weeks and observed that there were periodic changes in testicle size, with an average interval between peak points of 36.5 days. We also examined pain thresholds in these animals by measuring paw withdrawal temperatures (PWTs) in the dorsal hind paws once a week for 22 weeks. Similar to the testicles, PWTs underwent periodic changes with an average interval between peak points of 27.6 days. Although their cycles were not synchronized, the mean values of testicle size and PWTs showed a significant correlation (P = 0.019). These results demonstrate the presence of male sexual cycle and indicate the relationship between testicle size and pain thresholds in cynomolgus monkeys, providing new insights into research not only in primates, but also in other continuous breeders, including humans.