A. Excitement. For men, this involves erection of the penis, which usually occurs within a few seconds after sexual stimulation begins. In addition, skin ridges of the scrotum smooth out and the testes are drawn towards the body. B. Plateau. In men, the diameter of the head of the penis increases slightly and deepens in color because of increased blood flow. Vasocongestion also causes the testes to swell, becoming 50 to 100 percent larger than in the unstimulated state. In addition, small amounts of clear fluid may sometimes be secreted from the Cowper’s gland and appear from the male urethra. C. Orgasm. Men ejaculate semen during this stage, which actually has two parts: First, men experience a sensation of ejaculatory inevitability, which occurs just before ejaculation begins. Second, contractions of the urethra, penis and prostate glad serve to expel semen from the head of the penis. D. Resolution. Immediately after ejaculation, men enter a refractory period, during which further orgasm or ejaculation is not possible. This time varies greatly between men and increases with age. A partial loss of erection occurs during the refractory period. Eventually, blood flow to the genitals returns to pre-excitement levels. (Masters and Johnson, 1966). For thirty years this model has served as the foundation for the study of human sexual response and sexual dysfunction. In the 1970s, Kaplan (1974) and later Lief (1977) identified a subset of individuals whose sexual dysfunction did not lie within the four stages described by Masters and Johnson (1966). Instead, these people experienced little interest in sexual activity. This fifth stage, theoretically occurring prior to those described by Masters and Johnson, was labeled sexual desire (Figure 1). Kaplan (1974) and Lief (1977) observed that patients with desire problems complained of an inability to become aroused, lacked interest in sexual activity, and even avoided sexual functioning. 11.