The activity and functioning of the HPA axis was investigated in red deer stags under basal and stress induced conditions. In order to overcome the confounding effects of stress inherent in standard methods of blood collection, a portable remote blood sampling device (“DracPac”) was developed and tested. Stags were blood sampled remotely during restraint in a mechanical crush and subsequent recovery at pasture. Cortisol, haematocrit, glucose and lactate levels were elevated during restraint and declined significantly following release to pasture. The DracPac device was used to deliver i.v. a range of ACTH₁₋₂₄ doses (1-64 IU/100kg) and collect blood for cortisol determination from red deer stags. ACTH significantly elevated plasma cortisol concentrations, with mean peak concentrations occurring 20 to 40 min post-infusion. Duration of the response (ranging from 80 to 160 min), was dose dependent, whereas cortisol concentration plateau with doses above 4 IU. Seasonal changes in the activity and responsiveness of the adrenal gland in red deer stags were quantified by measuring 24 h endogenous cortisol secretory profiles and plasma cortisol responses to either administration of exogenous ACTH or a standardised stressor during November (period of velvet growth), February (pre-rut), April (mid-rut) and July (post-rut) (southern hemisphere) using a remote blood sampling device (DracPac). Ultradian, circadian and seasonal rhythms in the concentration of plasma cortisol were observed. Mean 24 h plasma cortisol concentrations were significantly higher in November than at other times of year. Peak cortisol concentrations following infusion of ACTH₁₋₂₄ (0.04 IU kg⁻¹) were also higher (p < 0.05) in November but lower (p < 0.001) in April than at other times of year. The role of gonadal steroids in modulating the seasonal rhythm of HPA axis activity and function in the red deer stag was investigated. Basal secretion of cortisol and responses to administration of ACTH₁₋₂₄ (0.04 IU kg⁻¹ live weight), CRH (25 ng kg⁻¹ live weight) and dexamethasone (37 μg kg⁻¹ live weight) were compared in castrated (n=6) and entire (n=6) male deer during the breeding season. A higher level of HPA axis basal activity and adrenal responsiveness to ACTH was seen in castrated compared with entire stags indicating that modulation of HPA axis function by testicular steroids occurs primarily at the level of the adrenal. The effect of chronic social stress on behaviour and hypothalamic-pituitary-adrenal axis (HPA axis) activity and function was determined in 6 two-year-old red deer stags (Mixed) subjected to repeated mixing into unfamiliar herds. Following mixing basal cortisol concentrations declined, the proportion of unbound cortisol increased though free cortisol concentrations did not change. Responsiveness of the adrenal to ACTH declined, as did pituitary responsiveness to CRH. HPA axis changes to stress occur at multiple levels within the axis. Acute stress is best assessed by frequent sampling of cortisol concentrations. Chronic stress can be best monitored by quantifying changes in HPA axis function (e.g. CRH challenge).