Anyone wishing to experiment with sex hormones will do well to review the literature recorded in the bibliography.
 The results so far reported in brief are these:
 Feeding the female hormone (oestrone) to both sexes of fry and older guppies, hindered growth and depressed the growth of females. In males the size at sexual maturity was increased. In older males their growth was enhanced but male coloration was lost.
 Feeding the male hormone (testosterone) depressed the growth of all young fry. Growth of partly grown females was enhanced.
 In yet another study, pregnenalone was fed in tablet form. It fulfilled the expectancy for an androgen (a hormone which controls the sex characteristics of males) better than testosterone proprionate. When fed to immature guppies from birth on, it completely prevented development of female secondary sex characteristics and caused early development of male secondary sex characters. It induced earlier-than-normal development of germinal elements and when given over a long period of time, destroyed the testes.
 Testosterone had similar effects but not as marked. In males it inhibited male red colors from developing, just as female hormones had been found to do.
 Thyroxine, a thyroid gland product, produced, in one study, larger guppies which required a longer growing period to mature. In males the thyroxine-treated individuals became about one-half larger but continued growing for nearly twice as long. In another study of putting synthetic thyroxine into the water, and feeding powdered thyroid gland, no changes could be observed.