Histochemical investigations concerning the activity of several dehydrogenases and hydrolases in the area cinguli during postnatal ontogenic development of the rat were performed. Brain sections corresponding to the region of area cinguli, obtained from rats aged 1, 3, 8, 17, 40 and 60 days postnatal, were subjected to histochemical assays for various dehydrogenases, phosphatases and esterases. The developmental changes concerning histoenzymic reactivity of the callosal gyrus with regard to several respiratory and hydrolytic enzymes were assessed and described. Considerable differences in enzymic reactivity, appertaining particularly to the phosphatases and esterases, between the anterior and posterior parts of the callosal gyrus were found. The unusual enzymic reactivity of glial cells in the individual regions of the area cinguli has been pointed out.