1. Experimental investigations on the influence of photoperiod and temperature on gonad development in Gasterosteus aculeatus were carried out between October 1967 and January 1968. 2. A combination of long photoperiod and high temperature has proved to stimulate gametogenesis while a combination of short photoperiod and high temperature leads to a complete cessation. Low temperature leads to a slight gonadal development, irrespective of day length. There seem to be no sexual differences in the reaction to these environmental factors. 3. The secondary sex characteristics of the male may appear despite total absence of interstitial tissue. This indicates that either testosterone may be produced outside the Leydig cells, or other hormones may be responsible for the appearance of the secondary sex characteristics. 4. Very high population density (1 fish/4 l) prevents the appearance of the secondary sex characteristics even in the presence of Leydig cells. The females are incapable of completing gonad development if the males do not exhibit secondary sex characteristics.