Prenatal undernutrition is known to have a significant impact on offspring organ development and a long term programming effect on subsequent health of the offspring. Although the brain is relatively protected from the effects of undernutrition, there are significant impacts on brain development (e.g. Feoli et al. 2006). In this study we investigated 1) whether prenatal undernutrition had a functional impact on the neonatal behavioural development in mice; and 2) whether the offspring of mouse lines selected for different growth traits would be disproportionately affected. Mice (n=54) of three lines: fast growing, fat (FF), fast growing, lean (FL) and normal growth, lean (NL) were assigned to a nutritional treatment on day 1 of gestation and were fed either ad libitum (C) or pair-fed to a weight-matched C dam at 80% of ad libitum (R). From birth onwards all dams were fed ad libitum, and three pups from each line were cross-fostered onto each dam within treatment. One male and one female pup from each line per litter (n=324) were assessed daily for physical development (pinnae detachment, tooth eruption, eye opening) and the development of reflex responses and motor skills until weaning at d16. The effects of genetic line, foster dam line and treatment were analysed by linear mixed models. R dams gave birth to smaller litters than C dams (C=16.8, R=15.4 pups, s.e.d.=0.53, P<0.05), and to lighter pups (C=1.74 g, R=1.44 g, s.e.d=0.05, P<0.001). Pups of FL dams were disproportionately lighter in comparison to other lines (26.1% reduction vs. 16.2% in FF litters, and 7.5% in NL litters, P<0.01). Pup physical development was significantly delayed in R pups for all measures (e.g. day of teeth eruption: C=10.30, R=10.69, s.e.d.=0.08, P<0.001). Forepaw grasp and placing reflexes were delayed in R pups, but these reflexes were accelerated in hindpaws in R compared to C pups. The appearance of other reflexes (vibrissae placing, righting, cliff drop response, negative geotaxis and climbing) were all delayed by approximately 0.5d in R pups compared to C (e.g. day maximal righting first seen: C=5.90, R=6.58, s.e.d.=0.19, P<0.001). In addition R pups tended to be slower to start crawling (P=0.08) and were slower to walk (day walking first observed: C=10.12, R=10.64, s.e.d.=0.14, P<0.001) than C pups. There were no significant interactions between pup line and treatment, or effects of foster dam on neonatal behavioural development. The data suggest that prenatal undernutrition causes a significant developmental delay in pup physical and neurological development, which is not affected by neonatal nutritional rehabilitation. Although mouse lines selected for fast lean tissue growth appeared less able to buffer foetal growth from the effects of gestational undernutrition, this did not affect neonatal behavioural development. Acknowledgements: This study was supported by SEERADReference 1 : Feoli AM et al. (2006) Nutrition 22, 160-165