The main objective of this study was to analyse the effects of r-met-hu G-CSF application as an inductor of early sexual development in prepubertal ram lambs. 20 Corriedale ram lambs (age= 2 months; ~18 kg l.w.) were divided in 4 groups: a control group (T1; n= 5) and 3 treated groups (T2, T3 and T4; n= 5 each) were administered a subcutaneous injection of r-met-hu G-CSF/Filgrastim daily for 4 consecutive days [(T2; 1 µg/kg l.w.; 18 µg/day; total= 72 µg), (T3; 10 µg/kg l.w.; 180 µg/day; total= 720 µg) y (T4; 20 µg/kg l.w.; 360 µg/day; total= 1.440 µg)]. A haematological analysis (white blood cells) was performed to determine the response of the spinal cord and confirm the hematopoietic effect of r-met-hu G-CSF. The testis weight, testicular morphometry and testicular volume were evaluated as well. To determine the effectiveness of the r-met-hu G-CSF treatment on sexual development, histological alterations (seminiferous tubules, Sertoli cells, gonocytes, spermatogonia, among others) were also evaluated as well as the endocrine response (FSH, LH, testosterone and cortisol) by ELISA. Significant differences were found in the number of leukocytes between the T4 vs. T1, T2 and T3 groups, from day 3 onwards (p<0.05). Lymphocyte percentage was significantly higher in T3 and T4 (p<0.05) compared to control (T1) and T2 from day 2 onwards. In addition, significant differences were observed in the testicular weight between the T1/T2 and the T3/T4 groups (p<0.05), as well as between the T1 and the T3/T4 regarding testicular volume (p<0.05). The hormone levels differed significantly in FSH concentration obtained in T3 compared to the other groups from day 3 onwards (p<0.05). Regarding LH and testosterone, significant differences were found between the T4 and the rest of the groups, which were evident from day 2 (LH) and day 3 (testosterone) (p<0.05). The cortisol did not differ among groups (p>0,05). With regard to histological alterations, T3 showed a more advanced cellular profile presenting primary spermatocytes, while T4 reached the generation of round spermatids. In conclusion, the application of r-met-hu G-CSF (Filgrastim) as a stimulating factor for early sexual development affected fast and positively the white blood cells´ haematopoiesis and the hormonal activity when applied at high concentrations, and accelerated the testicular development in prepubertal lambs, affecting not only endocrine hormone levels but also the testicular weight and volume, as well as the spermatogenic process.Keywords: r-met-hu G-CSF, testicular histopathology, sexual development, hormonal profile, ovine RESUMENEl principal objetivo del presente estudio fue analizar el efecto de la aplicación de r-met-hu G-CSF como inductor al desarrollo sexual temprano en corderos prepúberes. Se utilizaron 20 corderos de la raza Corriedale (edad: 2 meses; ~18 kg p.v.) divididos en 4 grupos: grupo control (T1; n= 5) y 3 grupos tratados (T2, T3 y T4; n= 5 cada uno) con una dosis diaria de r-met-hu G-CSF/Filgrastim durante 4 días consecutivos [(T2; 1...