Aminopterin was found to depress the hypertrophy of the oviduct of immature pullets induced by estrogen plus androgen. The diet was arranged to be low in folic acid during the experimental period. Higher dosage levels of aminopterin did not further decrease the size of oviduct in estrogenized pullets below a plateau level which was higher than that of the unestrogenized controls. Aminopterin opposed the effects of the hormonal treatment on liver weight per kgm. live weight, adrenal weight, and spleen weight. Injections of folic acid not merely antagonized this effect of aminopterin, but increased the size of the spleen above that of the estrogenized controls. Intraperitoneal injections of desoxyribonucleic acid also opposed the effect of aminopterin on the spleen of the estrogenized pullet. Aminopterin appeared to have no effect on liver phosphoprotein P of the estrogenized pullet.Neither aminopterin nor aminopterin plus desoxyribonucleic acid affected serum calcium level significantly, but aminopterin plus folic acid significantly lowered the serum calcium level of the estrogenized pullet in one experiment. The differences between the groups in respect of liver nucleic acid content were slight and did not attain statistical significance.