The influence of testosterone on libido has been widely accepted in spite of clear indications that libido is also influenced by psychosocial factors, which are related for example by cultural and religious factors. Recent research has provided evidence that a decreased libido may be caused by 1. a highly sensitive brain for sexual cues and strong psychological inhibition, and 2. by a low sensitive brain for sexual cues. This distinction shows two different fundamental mechanisms to increase sexual desire. Firstly, activation of 5-HT1A receptors in the frontal lobe diminishes psychological inhibition. Secondly, increasing genital NO contents increases libido on the condition that the brain has been primed 4 hours before with a low dosage of testosterone. Understanding of both mechanisms is essential for a better understanding of patients with libido difficulties.