In connection with a problem on endocrine-influenced behavior, supported by National Institutes of Health Grant MH-969, a large colony of Wyoming ground squirrels, Citellus richardsonii elegans has been maintained for several years in our vivarium. On April 9 newly captured female No. 610 was found to have a cornified vaginal smear and was placed in our standard observation cage with an active healthy male squirrel at 10:50 a.m. He immediately approached the female and sniffed her nose, backed off and shoved sawdust toward her, reapproached and nibbled her neck and upper back. She now elevated her tail and chirruped. The male nuzzled her genital region and mounted the female at 11:08 a.m. The female whistled shrilly but did not resist. The male clasped the female from behind with his fore paws about her lower thorax. There was some palpation by the male such as is...