Seventy men, aged 20-40 years, who were attending an infertility clinic were studied to evaluate associations between seminal quality, whole blood serotonin, and serum testosterone levels. Men with blood serotonin levels > 100 ng/ml showed lower sperm counts and sperm motility than those with normal levels of blood serotonin (<90 ng/ml). Seminal volume, pH, sperm morphology, fructose, citric acid, and serum testosterone values were similar between groups of patients with different levels of blood serotonin. The levels of blood serotonin correlated best with sperm motility when using a cubic regression analysis. When blood serotonin increased from 50 to 90 ng/ml (normal range), there was an increase in the percentage of spermatozoa with excellent motility. However, when serotonin was above 90 ng/ml the percentage of sperm with excellent motility decreased. An inverse exponential relationship was observed between blood serotonin and sperm count. The sperm count reached values of oligozoospermia when blood serotonin was over 100 ng/ml. It would appear that blood serotonin in the normal range is important for sperm motility. Values of serotonin above the normal range could affect negatively both sperm count and motility. These effects were observed in the absence of changes in serum testosterone levels. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.