The effects of pH and osmolarity of semen and urine specimens on motility and velocity of human spermatozoa were studied objectively with the aid of the multiple exposure photography (MEP) method. The pH of fresh ejaculates ranged from 7.2 to 8.2 and specimens were slightly hyperosmotic ranging between 300 to 380 mOsm/kg. Gradually changing the pH and osmolarity to either side of normal values led to progressive loss of sperm motility. However, sperm velocity was slightly increased by mild alkalinization and hyperosmolarity. Spermatozoa that became immobilized by acidification regained their motility shortly after pH was restored to normal values. In the majority of instances spermatozoa lost their motility when mixed with fresh urine specimens. Neutralization of urinary pH could not protect them from this effect unless urine osmolarity was also isotonically adjusted. It is suggested that patients with retrograde ejaculation should adequately increase their fluid intake before recovery of sperm from their bladder for artificial insemination.