The experiment examined the effect of body weight, season of birth and GnRH injection i.v. (5 ug/kg body weight/animal 2-h intervals for 24 hours) on age at puberty and p4 concentration in 10 Rhamni ewe lambs six months of age with mean body weight 19.88 ± 0.78 kg. This study started on June 20, 2001 and continued until the expected time of puberty in the Farm of Animal Production Department, Faculty of Agriculture, Al-Azhar university, Assiut. The ewe lambs were divided at random into two equal groups. The first group included three lambs born in autumn and two ewe lambs born in winter and this group was considered as control (Ctrl). The second group also involved three ewe lambs born in autumn and two ewe lambs born in winter and this group was treated by GnRH i.v. injection and considered as treated ewe lambs (Tr.). After GnRH injection , two fertile rams were allowed to remain with each group for half an hour/day for estrous detection. All ewe lambs were fed the same ration with 11 % crude protein and 12 % ash. Each animal was given 750 g dry matter/daily of the ration which consisted of yellow corn , wheat straw, wheat bran and decorticated cotton seed meal. Green fodder, water and mineral blocks were freely available throughout the experiment. Blood samples were collected from the ewe lambs starting from June 20,2002 and the expected time of puberty . Blood (5 ml) was drawn from the jugular vein twice a week. Blood serum was separated and stored at -20 oC until assayed for P4. When the P4 concentration reached the peak for the first time over 1.0 ng/ml blood serum, this was taken as the occurrence of puberty. Results indicated that the difference between body weight of ewe lambs treated with GnRH and the Ctrl. Lacked significance. The overall mean of Rahmani body weight at puberty represented 38-42 % of the adult body weight . There was no significant difference between the two groups in age at puberty. Most treated lambs displayed higher P4 concentration than control lambs. Season of birth had also a highly significant effect (P0.01) on P4 concentration. The first peak of serum P4 (over 1.0 ng/ml) concentrations occurred 99 days after the beginning of GnRH treatment, while the second peak of over 1.0 ng/ml) concentration occurred 90 days after the first peak of P4. Treated lambs born in winter achieved the first peak of P4 concentration much earlier than control lambs born in autumn. So, GnRH was more effective in lambs born in winter than those born in autumn in accelerating the time of puberty in Rahmani ewe lambs