Long-term (annual and seasonal) variations in plasma concentration of reproductive hormones (maturational gonadotropin GTH and sex steroids) have been studied in relation to the gonad cycle in some teleosts. Plasma GTH concentrations are low and increase only gradually during the major part of gonad development (vitellogenesis, spermatogenesis) but increase sharply towards the end of gametogenesis–that is, around the time of oocyte maturation and ovulation and before the start of spermiation. In females, plasma concentrations of 17β-estradiol and estrone rise continuously during vitellogenesis. In salmonids, after a decrease in 17β-estradiol concentration starting at the end of the vitellogenesis, there is a small peak in testosterone concentration followed by a strong transient rise in 17α-hydroxy-20β-dihydroprogesterone, which induces oocyte maturation. The concentrations of testosterone and 11-ketotestosterone rise during spermatogenesis. Maximal concentrations of 11-ketotestosterone were recorded in rainbow trout Salmo gairdneri just prior to the start of spermiation. Plasma concentrations of both androgens in this species dropped during spermiation, whereas that of 17α-hydroxy-20β-dihydroprogesterone showed an important rise. Short-term (circadian and ultradian) variations in concentrations of reproductive hormones in teleosts have been less studied. Daily fluctuations in plasma GTH and in sex steroid concentrations, the pattern of which depends on the development stage of the gonads, have been demonstrated in a few fish species. Pulsed gonadotropin secretion occurs in some phases of gametogenesis in rainbow trout and common carp Cyprinus carpio.