This study reports the effects of peripubertal GnRH receptor inactivation ondevelopment of psychophysiological motoric reactivity (PMR; sometimes also called emotionalreactivity), plasma cortisol concentrations and the relationship between plasma cortisol and PMRin male and female sheep.The study formed part of a larger trial and utilised 46 same sex twins. One twin remaineduntreated (control) while the other received a subcutaneous GnRH agonist (GnRHa Goserelin-Acetate) implant every 4th week, beginning at 8 and 28 weeks of age, in males and females,respectively (different, due to sex specific age of puberty). PMR, a measure of an animals’response to social isolation, was measured over a two minute period at 8, 28 and 48 weeks of age,using a three axis accelerometer. During the test period vocalisation rate was recorded. Cortisolwas assayed in blood samples collected on a single day when animals were 40 weeks of age.PMR and vocalisation rate were significantly higher in females than males at all ages tested. At28 weeks of age (20 weeks treatment) PMR was increased in treated males to the level seen incontrol females, by 48 weeks of age treated males’ PMR was significantly less than controls. Infemales, 20 weeks of GnRHa treatment (28—48 weeks of age) was not associated with differencesin PMR. Cortisol concentrations were significantly higher in females than males but were not affected by treatment. Plasma cortisol concentrations were positively correlated with PMR;this relationship being driven by the treated animals in both sexes.The results demonstrate that PMR is sexually dimorphic and cortisol dependent in sheepfrom at least 8 weeks of age. Importantly, they also demonstrate that long-term treatment ofmales with a GnRH agonist results in changes in age-dependent development of PMR.