Methylene blue (MB) is a thiazine dye used in the treatment of methemoglobinemia. Since it was shown to scavenge free radicals, it is now being examined clinically in reperfusion syndrome and septic shock. We tested methylene blue in a series of experimental endocrine situations, in which this scavenging effect could play a role. Indeed, we observed that MB partly inhibited the increase in adenohypophyseal weight and cAMP and prolactin levels after the administration of estrogens in male rats. MB also inhibited the increase of another scavenger of free radicals, the metalloenzyme ceruloplasmin in the blood of estrogenized rats. MB also inhibited the stimulation of bone mass after estradiol in male rats. In this respect, it behaved as an antiestrogen. In the bones, MB also prevented the increase of bone minerals induced by estradiol. MB also produced a decrease in adenohypophyseal ascorbic acid content an potentiated the effect of estradiol in the same direction. Surprisingly, blood thyroxine levels increased consistently in rats after MB treatment. An interaction of MB with thyroid hormone synthesis and/or actions thus should be evaluated. This idea is supported by our observation of an antagonistic action of MB in carbimazole-induced rise in thyroid weight and decrease in thyroxine blood levels. (Ref. 14.).