The role of androgens and insulin-like growth factor 1 (IGF-1) in antler growth has been disputed. We predicted that the secretory of IGF-1 may be associated with an acceleration of body growth rather than with antler growth. Furthermore we anticipated a relationship between the increase of testosterone and the progress of antler growth. If IGF-1 is involved in the stimulation of antler growth, this should be more obvious in young than in mature stags. Eight two-year-old red deer stags (Cervus elaphus), and twelve adult red deer stags were blood sampled and the length of their velvet antlers was measured in one-week intervals during the period of antler growth. Concentrations of testosterone, cortisol, IGF-1, luteinizing hormone (LH), and prolactin were determined in plasma by enzyme immunoassay or radioimmunoassay. Antler growth per day was primarily dependent on changes in testosterone concentration per day in both groups of stags.