[1-(L-2-Hydroxy-3-mercaptopropanic acid)]arginine-vasopressin (hydroxy-AVP), [1-(L-2-hydroxy-3-mercaptopropanoic acid),4-valine,8-D-argine]vasopressin (hydroxy-VDAVP) and were synthesized by a combination of the solid-phase and solution methods of peptide synthesis. Protected octapeptides synthesized by the solid-phase method were further acylated by 1 + 8 couplings in solution to furnish the key intermediates. Hydroxy-AVP has antidiuretic potency of 470 units/mg and activity in the rat vasopressor assay of 550 units/mg, representing a small enhancement of activity over that of arginine-vasopressin (AVP) in each case. Hydroxy-DAVP and hydroxy-VDAVP have essentially the same high antidiuretic activity (900 units/mg) and very low vasopressor potencies (0.9 and < 0.02 units/mg, respectively). Hydroxy-AVP, hydroxy-DAVP and hydroxy-VDAVP have antidiuretic-pressor selectivity (A/P) of 1, 1000, and > 45,000, respectively. These data are compared with those of other vasopressin analogs. Hydroxy-VDAVP is a highly specific antidiuretic peptide and may be useful in pharmacological studies of antidiuresis.