A Fsample of young 60 hens and 40 roosters of Rhode Island breed were divided in groups L1, L2 and L3. Individuals in group L1 were transferred at the age of 16 weeks from 8 h light/ day to 16 h light/ day and fed with a normal adult poultry feed. The same switch was performed with groups L2 and L3 at the age of 18 and respectively 20 weeks. Modifications of the sexual traits were recorded including: phenotype, weight and length of the genital tract, laying intensity, average egg weight and

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... n parameters. Furthermore, blood samples were analysed for FSH and LH. From photo stimulated roosters -kept under an 8 h light/ day programme at the age of 18, 22 and 24 weeks- histology samples from testis and deferens ducts were analysed. In young hens reproductive parameters are influenced both by light length and the age when this photo stimulation occur. All data shows that starting photo stimulation at 18 weeks has positive effects on the egg production in the analysed population. In young roosters inducing sexual stimulation before the age of 20 weeks will prolong the time length to typical reproductive activity and also will affect the semen quality. Thus, it seems that age is more important for young roosters in achieving reproductive maturity than in young hens. Therefore, inducing early sexual development is rather detrimental for males.