In a longitudinal study one healthy man aged 26 yrs underwent semen collections and blood samplings at monthly interval from January to December 1985. The ejaculated samples were analyzed for sperm density, motility, relative proportion of early (ab) and late (cd) spermatids, concentrations of Fructose, L-Carnitine and Transferrin. In a transverse study, 248 serially independent samplings were performed of normal men, 28.9 +/- 7.5 yrs old, consulting for infertility from January 1981 to December 1983. The Cosinor method was employed for searching on circannual rhythms of the studied parameters. The longitudinal study did not detect circannual rhythms of the considered seminal parameters nor of the studied hormones. However, peaks, significantly above the mean values throughout the year (p less than 0.005), were observed of Cortisol, PRL, FSH, LH, Testosterone, Fructose and L-Carnitine levels. Young spermatids and sperm motility showed significant (p less than 0.001) acrophases between June and July; an acrophase was also evident for late spermatids but shifted by six months, between December and January. Mesors and amplitudes were respectively: 28 +/- 1.5 and 9.6 +/- 4.3 (mean +/- SE) for motility; 33.4 +/- 1.6 and 9.9 +/- 5 for early spermatids; 46.6 +/- 1.9 and 13 +/- 6.2 for late spermatids.