Figure 6. The Role of Testosterone in Regulation of Sexual Function This schematic diagram illustrates androgen-dependent and androgen-independent components of the male sexual function. The darkly shaded boxes include items for which there is evidence of androgen-dependence. The lightly shaded boxes contain items that are believed to be androgen-independent. Although libido is loosely used interchangeably with sexual desire, it is a more complex function that includes generation of spontaneous sexual thoughts and fantasies, attentiveness and responsiveness to erotic stimuli, and pleasure seeking behavior. While there is evidence that testosterone stimulates spontaneous sexual thoughts and fantasies, attentiveness and response to erotic stimuli, we do not know whether arousability and pleasure seeking behavior are also stimulated by testosterone. Penile erections are largely the result of biochemical processes within the cavernosal smooth muscle that result in cavernosal smooth muscle relaxation and increased blood flow. Penile erections can occur in hypogonadal men, particularly in response to appropriate erotic stimuli. However, there is growing body of evidence that testosterone might induce nitric oxide synthase within the cavernosal smooth muscle and might be necessary for achievement of optimal penile rigidity. Orgasm and ejaculation are androgen-independent and can occur in the absence of a full erection.