The objective of this study was to determine if the continuous treatment of young rams with an agonist of gonadotropin-releasing hormone (GnRH) in the period immediately prior to puberty would delay the onset of adult sexual behavior and retard testicular development. In the first experiment the GnRH agonist was shown to be effective in suppressing the plasma concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in adult wethers (neo-natally castrated rams) when administered by either a biocompatible slow release implant (implant) or a mini osmotic pump (minipump) that released the agonist for 4 weeks. The minipumps were more effective than the implants in suppressing the secretion of LH and FSH. In a second experiment, administration of the GnRH agonist by implant or minipump to prepubertal rams for 16 weeks immediately prior to puberty inhibited the development of sexual behavior, reduced the plasma concentrations of testosterone, retarded testicular and epididymal development, and inhibited growth rates. The effects on sexual behavior were clearly reversible but testicular and epididymal weights were still reduced in treated rams 8 weeks after the end of treatment. These results indicate that the reproductive function of rams is sensitive to gonadotropins and testicular hormones immediately prior to puberty. The agonist of GnRH was successfully delivered to the rams in a biocompatible implant which may offer a practical means of manipulating reproductive function in young rams.