Sperm concentration (A) and motility (B) in 8 healthy men during 6 months of melatonin-placebo administration. The solid line represents the lower normal limit suggested by World Health Organization guidelines (1993). The sequence of medications was a placebo followed by melatonin for subjects 1–4 and melatonin followed by a placebo for subjects 5–8. All 8 subjects had a 2-week washout period after the first 3-month treatment period. Statistically different values are indicated with an asterisk.