Gonadotropin releasing hormone (GnRH) exerts its action by binding to the specific receptor which belongs to the family of G-protein coupled receptors that are characterized by the presence of seven transmembrane domains linked together by extracellular and intracellular loops. A fragment of the pituitary receptor of the bonnet monkey (Macaca radiata) corresponding to amino acids 164–266 was cloned and expressed in Escherichia coli. This was used to raise antibodies to the receptor in rabbits. Active and passive immunization studies in both male and female rats were carried out using, both the `overexpressed' fragment, as well as antibodies raised to the receptor fragment. Both active, as well as passive immunization in the male rats brought about an agonistic effect in terms of increase in serum LH level, as well as increase in serum and testicular testosterone levels. However, in the female rats, active immunization with the receptor fragment did not have any effect on the gonadal steroid levels but had a selective effect on the uterine morphology.