Fish display evident changes during the breeding season, for example, the males of certain species show an expansion of the chromatophoric skin cells with a usual production of more brilliant coloring over certain body parts. This mating coat or 'wedding dress' is especially noticeable in fish of the Cyprinidae group. The female bitterling during breeding season shows an elongation of the external ovipositor and this change is readily discernible.These breeding season changes in fish have been produced artificially by a variety of substances and more particularly the hormones. Wunder (1) using the male bitterling {Rhodeus amarus) obtained positive results with testis extracts, varying results with prolan and no results with female sex hormones. Tozawa (2) noted that the male Japanese bitterling (Achelognathus intermedium) developed chromatophore reactions to hormones injected during the breeding season (April 15 to July 1). Zondek (3) used the erythrophore expansion of Phoxinus laevis in demonstrating the pituitary pigment hormone intermedin. Fleischmann and Kann (4) employing the European bitterling {Rhodeus amarus) found progynon effective in causing ovipositor lengthening in the female. Saline solutions and the prolan hormones proved ineffective. They also implanted bitterling ovaries into female rodents and obtained estrus, thus apparently demonstrating the presence of nonspecific ovarian hormones in the fish ovaries. Szusz (5) using similar fish as a possible test subject in the determination of human pregnancies, presented results indicating the use of fish in these tests to be of little clinical value. He also found discordant results with anterior pituitary extracts, male urines and with urines from non-pregnant females. Glaser and Haempel (6) have reported that the castrated bitterling may be useful as a test animal in demonstrating the male sex hormone. Kanter et al (7) recently renewed interest in the bitterling test for pregnancy when they reported 31 such tests, 27 of which were also positive according to the Friedman assay. Kleiner et al (8) repeated the work of the above authors and concluded as did Szusz (5) that a pregnancy test employing fish was extremely unreliable. Saphir (9) employing subcutaneous injections in the male red bellied dace {Chrosomus erythrogaster) found prolan to act inconstantly, yohimbine hydrochloride to be effective but that saline solutions, the urines of males, females or of pregnant women as well as adrenalin, progynon, theelin, testis hormone and pituitary extracts were ineffective in producing the typical mating coat.