Based on a review of the pertinent literature and our own unpublished
data, it is recommended that culling of rodent litters in the early po
stnatal period should be a standard practice in delivery-type reproduc
tion studies. This, in turn, will reduce the litter size-induced varia
bility in the growth and development of pups during the postnatal peri
od and thus increase the sensitivity of statistical analyses to detect
treatment-related effects. This will also ensure that any adverse eff
ects on pup growth (body weight gain) and development (reflex and beha
vior development) are not masked by a treatment-induced reduction in l
itter size. The culling should be carried out randomly and no attempt
should be made to selectively cull sick or underweight pups. Since mal
e pups weigh significantly more than females and studies have shown di
fferences in maternal behavior toward one sex over the other, whenever
possible each culled litter should consist of an equal number of male
s and females. (C) 1997 Society of Toxicology.