The adenohypophysial primordium of Xenopus laevis tadpoles at stages 33/34 to 46 (Nieuwkoop and Faber, 1956) were examined immunohistologically for alpha-MSH, beta-MSH and ACTH. beta-MSH was demonstrated from stage 37/38 onwards, and alpha-MSH from stage 39. No signs of ACTH production were detected. alpha-MSH and beta-MSH occurred in the same cells. No differences were found in the intensity of immunofluorescence between tadpoles which were kept on a black and a white background. The present study lends no support to the hypothesis concerning the derivation of alpha-MSH from ACTH. The observations made suggest that the morphological formation of the pars intermedia is accomplished during stages 37/38 to 39.