The rates of incorporation of [35S]cysteine into arginine vasopressin (AVP) and oxytocin (OXT) were studied concurrently in hypothalami from intact and hypophysectomized male rats. After label injection into the third ventricle, rats were killed 0.5, 1, 2, 4, or 8 h later. The hypothalamic peptides were quantitated by specific RIA and separated by HPLC to allow quantitation of label incorporation into each peptide. In intact rats, labeling of both OXT and AVP rose rapidly to peak at 2 h; thereafter, radioactivity in both peptides declined slowly to 8 h. In hypophysectomized rats, labeling of AVP fell below that in intact animals at all time points. Labeling of OXT was somewhat below normal 2 h after label injection, but considerably above normal at 4 and 8 h. For comparison, label incorporation into somatostatin-14 (SRIF-14) and somatostatin-28 (SRIF-28) was also studied in the same animals. In intact rats, labeling of both SRIF peptides rose slowly to maximal values at 8 h. In hypophysectomized animals, labeling of each peptide was reduced substantially at all times tested. Endogenous cysteine specific activity and protein specific activity did not differ between intact and hypophysectomized animals. Immunoreactive levels of AVP and OXT in the hypothalamus were unaffected by hypophysectomy, though total SRIF-like immunoreactivity was depressed. Together these results suggest that hypothalamic neurons synthesize OXT and AVP at rates much faster than those for the SRIF peptides, and that hypophysectomy has differential effects on the syntheses of cysteine-containing peptides in the hypothalamus. Specifically, the syntheses of SRIF-14 and SRIF-28 appear to be sizeably reduced in hypophysectomized rats, while that of AVP only modestly diminished. OXT synthesis may be increased.