More than 3000 mammals were captured and translocated during a rescue operation conducted in French Guiana during the filling of a hydroelectric dam, a unique opportunity to gather biological data on many aspects of the Guianan fauna. All animals were identified, anaesthetized, measured, and biological samples (blood, skin biopsies) were collected and preserved. This study presents morphological data on all captured mammals belonging to 40 species of Marsupialia, Xenarthra, Primates, Carnivora, Artiodactyla, Rodentia and Sirenia. Data reported here include weight, shoulder height, head and body, tail, hind foot, head, and ear lengths, and some additional measurements for particular species. Some species were found to be larger than previously reported, which may be attributed to the large sample size, circumstances of data collection or to geographic variations. Such basic data appear to be hardly available on neotropical mammals, and thus provide useful comparative material for further studies.