A plasmid delivery system validated in other species was assessed for its potential for inducing long-term expression of gonadotropin-releasing hormone (GnRH) in stallions. The efficacy of this technique was demonstrated using two plasmids: pSEAP, expressing secreted embryonic alkaline phosphatase (SEAP), and pGnRH, expressing GnRH. In experiment 1, geldings were used as a model to test the effect of muscle of injection (splenius, pectoralis, and semitendinosus; n = 3 for each site) on the expression of the reporter plasmid, pSEAP. Concentrations of SEAP rose ( P P P P P P > .1) of pGnRH treatment on seminal characteristics. It was concluded that delivery by electroporation of plasmids encoding peptide hormones may serve as a means of long-term in vivo production of peptides in the horse. Increases in LH and testosterone secretion after GnRH were observed in pGnRH-treated stallions; however, optimal conditions for expression need to be determined in future experiments.