National Toxicology Program

The NTP will use the Harlan Sprague Dawley as the rat model for NTP studies, in lieu of the Wistar Han rat. This decision came after much deliberation with NTP management. This change is necessary because of the reproductive capability of the Wistar Han and its size. Many NTP studies call for the use of pregnant rats with dosing commencing early in pregnancy. The NTP has used time pregnant Wistar Han rats for more than one year. During that time it has become evident that the reproductive robustness of the Wistar Han rat appears to wane when in a timed mating situation. Litters sizes are much smaller than the typical 10 pups/litter seen in natural mating situations. Because of smaller litter sizes and skewed sex ratios of the pups, the number of usable litters decreased significantly, negatively impacting the availability of animals for studies. Additionally, the large size of the Wistar Han rat from other vendors precludes their use in NTP studies. Information regarding procurement of Harlan Sprague Dawley rats will be provided as soon as it is available.