This study examined whether gender and/or training were related to the exercise-induced changes in plasma concentrations of growth hormone (GH), prolactin (PRL), triiodothyronine (T3), and thyroxine (T4). Twenty subjects (male and female 10-km runners; untrained males and females) ran on a treadmill for 30 min at 80% of previously determined maximum heart rate. Blood samples were taken through an indwelling catheter from an antecubital vein at -30, 0, +15, +30 min of the test and 30 min of recovery. Rectal temperature rose significantly (p < .01) at +15 and +30 min with concomitant rise in GH concentration, but PRL, T3, and T4 were not affected by the exercise. We concluded that a 30-min run at 80% of maximum heart rate is associated with higher concentrations of GH but not of PRL, T3, and T4. Neither training state nor gender affected the aforementioned results.