This study investigated effects of a 9-week intensified aerobic training and 3-weeks of recovery on signs of overload in 9 healthy active young males. Blood and saliva samples were collected and psychological questionnaires were administered during baseline (T1), intermediate load (T2), maximal load (T3), and recovery (T4) periods. Maximal oxygen uptake increased and blood lactate concentration decreased in T3, while running time in a 3 000  m track field test was significantly shorter. No significant changes were found in hematocrit, haemoglobin concentration, white blood cell count, lactate dehydrogenase, transaminases, interleukin-6, tumour necrosis factor-α, myeloperoxidase and markers of oxidative stress in plasma, or salivary cortisol and testosterone. Increases in different negative affect scales and in the total mood disturbance score of the Profile of Mood States were observed during T3. Scores in the stress scales of the Recovery-Stress Questionnaire for Athletes and in the State Anxiety Scale of the State-Trait Anxiety Inventory also showed significant increases during T3. The lack of effects in biomarkers together with the changes observed in psychological assessment indicates that an intensified training can produce psychological disturbances prone to early overreaching development. Additionally, it seems that psychological parameters are sensitive markers to detect stress produced by load increases.