[1-(L-2-Hdroxy-3-mercaptopropanic acid)]arginine-vasopressin (hydroxy-AVP), [1-(L-2-hydroxy-3-mercaptopropanoic acid),8-D-arginine]vasopressin (hydroxy-DAVP), and [1-(L-2-hydroxy-3-mercaptopropanoic acid),4-valine,8-D-arginine]vasopressin (hydroxy-VDAVP) 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 less than 0.02 units/mg, respectively). Hydroxy-AVP, hydroxy-DAVP, and hydroxy-VDAVP thus have antidiuretic-pressor selectivity (A/P) of 1, 1000, and greater than 45 000, respectively. These data are compared with those of other vasopressin analogues. Hydroxy-VDAVP is a highly specific antidiuretic peptids and may be useful in pharmacological studies of antidiuresis.