Desire is first stage of sexual response cycle (kaplan) Male sexual desire may be more frequent and intense than female sexual desire. Sexual desire varies within men and within women and over the life course. Excitement phase involves build up of vasocongestion and myotonia (muscle contraction), as well as positive or negative cognitive and affective appraisal or arousal experience. Vasocongestion: an accumulation of blood in the blood vessels of a region of the body, especially the genitals; a swelling or erection results. Erection: corpra cavernosa and corpus spongiosum fill with blood (smooth muscles relax, nitric oxide, pde5 inhibitors act on nitric oxide synthesis, affect erectile response to stimulation (not spontaneous; not aphrodisiac, partial elevation of testes. Plateau phase involves maintenance of high levels of physiological arousal and additional changes: testes increase in size and elevate, prostate enlarges, glans penis colors, pre-ejaculatory emission from cowper"s gland. Desire excitement plateau orgasm resolution.