Puberty is the phase of development during which the potential to reproduce is first realized. Puberty is associated with a variety of physiological and behavioural changes brought by testicular maturation and the production of adult levels of testosterone. These events are the result of re-activation of the gonadotropin releasing hormone (GnRH) neuronal system, which after a brief prenatal period of activation is quiescent throughout early postnatal life (Romeo et al., 2002).

The hypothesis that leptin plays an important role in regulating GnRH secretion, and ultimately in reproduction, stems from several studies. However, both sexual maturation and nutritional status are important determinants of how leptin affects the H-P-G axis in ruminants. Leptin is one of several permissive factors, whose presence is necessary but alone is not sufficient to initiate sexual maturation in rodents (Williams et al., 2002; Spicer et al., 2002). Furthermore, Garcia et al. (2002) reported that serum leptin increases linearly as puberty approaches in heifers, hence it is plausible to hypothesize that leptin may play a functional role in regulation of the central reproductive axis. The objective of this experiment was to study the changes of plasma leptin during puberty and its relationship with testosterone and testis dimensions in bull Holstein calves.