The working hypothesis was that restricted protein content in diet of prepubertal ewe lambs would delay time of onset of puberty to a greater extent than restricted energy content. Restriction of dietary protein (Metabolizable Protein: MP) or Energy (Metabolizable Energy: ME) on age and weight at puberty were assigned to one of the three dietary treatment groups: control (CTL; 18 MJ ME d-1 and 173 g MP d-1); protein restricted (PR; 11 MJ ME d-1 and 66 g MP d-1); energy restricted (ER; 10.2 MJ ME d-1 and 96 g MP d-1). Ewe lambs were weighed bi-weekly and bled weekly, and serum was assayed for progesterone to determine time of puberty. Lambs in the CTL group gained more than lambs fed the PR diet and lambs fed the PR gained more weight than ER group (P less than 0.05). Diet affected age (P less than 0.001) but not weight (P equal to 0.13) at puberty. Body weight gain at prepuberty is more important than type of dietary restriction (protein or energy) with regard to impacting age at puberty. We therefore reject the working hypothesis that dietary protein restriction at prepuberty would have a greater impact in delaying onset of puberty than dietary energy restriction.