A man's semen parameters may vary considerably from one specimen to the next, partly due to variability in the conditions under which the specimens are produced. In the present study, the relationship between the duration of preejaculatory sexual arousal and the quality of semen produced by masturbation was investigated. Twenty-five regular semen donors aged 22-44 provided a total of 292 semen specimens (median 11 per donor) over a period of 4 months. Each specimen was produced after a minimum of 3 days of ejaculatory abstinence and measures included the time taken to produce the specimen, ejaculate volume, sperm concentration, and percent motility. Linear regression revealed that, controlling for donor identity, there was a significant (t=2.13, P<.05) positive relationship between the time taken to produce a specimen and sperm concentration. We conclude that the duration of preejaculatory sexual arousal is an important predictor of ejaculate quality for specimens produced by masturbation and that variation in the duration of preejaculatory arousal may contribute to within-male fluctuations in semen parameters over time.