The activity of thiamine diphosphate phosphohydrolase (TPP-ase) was measured by a quantitative histochemical method in a histologically pure supraoptic nucleus (SON) and an adjacent area of the anterior hypothalamus (AH). During lactation, after castration, during early estrus and after LH administration, an increased TPP-ase activity (57-86%) was found in the SON, while no changes were observed in the AH. These results are indicative of an increased synthetic activity in the SON. Consequently, the activity of TPP-ase in the SON can be used as a parameter for posterior lobe hormone production. (Endocrinology 89: 1123, 1971) THE SUPRAOPTIC and paraventricular nuclei (SON and PVN) synthetize the posterior lobe hormones, vasopressin and oxytocin (for a review see ref. 1). In these nuclei the Golgi apparatus responds to osmotic activation by enlargement and changes in organization (24). In line with these morphological findings the spatial distribution of thiamine diphosphate phosphohydrolase (TPP-ase)—the marker enzyme of the Colgi apparatus (5, 6)—increased in the magnocellular nuclei (7). Moreover, an increased TPP-ase distribution was shown during other stimuli for the SON and PVN, i.e., during midpregnancy, parturition, lactation, after gonadectomy, during early estrus and light-induced persistent estrus, and after administration of gonadotropic hormones (8-10). In the present study the quantitative histochemical method of Lowry (11) was applied, since the localizing enzyme histochemical procedure only registers enzyme distribution, which does not implicate changes in enzyme activity. Both procedures have been used during osmotic activation of the SON, and showed the same results (7, 12). To compare both procedures also during other circumstances that activate the neurosecretory nuclei (8-10), the present study deals with the determination of TPP-ase activity during lactation, after castration, during early estrus and after gonadotropic hormone administration. Materials and Methods Wister rats (T.N.O., Zeist, approximately 200 g) were kept in individual cages at 25 C and exposed to 12 hr light daily (from 7 AM to 7 PM). They received tap water and standard chow (Hope Farms) Seven animals were killed at day 10 of lactation together with 7 control animals during metestrus. Using Hypnorm anesthesia (0.1 ml/100 g body wt, im), 7 male rats were castrated and 7 others were sham operated. All animals were killed 2 weeks after operation. Seven female rats were killed during metestrus together with 8 animals in early estrus (characterized by cornified smears together with heavy, distended uteri). Fourteen female rats were ovariectomized, using Hypnorm anesthesia. Starting on the day of operation all animals received 2 jig estradiol benzoate (Dimenformon) daily (0.1 ml, sc) during the entire experimental period. During the last 2 days before sacrifice 7 animals received 150 M-g LH 2 times daily (NIH-LH-S15). The animals were killed 2 hr after the fifth gonadotropic hormone injection. All animals were killed at 10:30 AM. The TPPase activity was determined (see Fig. 1) in dissected pieces of SON (approximately 0.3 M-g) and adjacent anterior hypothalamus (AH, approximately 2\ig), as has been described by Jongkind (12). Enzyme activities are expressed in moles of substrate hydrolyzed/kg dry weight of tissue during 1 hr of incubation at 30 C (MKH units). The results of 3 separate TPP-ase determinations in the SON during the particular experimental condition were combined according to De Jonge (13). The significance of differences between experimental and control groups was calculated using the Student Mest, while these results were distribution free checked by the Wilcoxon test. Received December 30, 1970. 1 LH was kindly provided by the Endocrinology Study Section, NIH.