Two series of 1,2,4-triazolo[4,3-a]pyrazine derivatives with human renin inhibitory activity have been synthesized which incorporate the transition-state mimetics (3S,4S)- and (3R,4S)-5-cyclohexyl-3,4-diaminopentanoic acid ((S)- and (R)-CDAPA), and (4S)-4-amino-5-cyclohexyl-2,2-difluoro-3-oxopentanoic acid (ACDFOPA). Several compounds in these series, for example 13a, 19c, and 19f, were highly potent inhibitors of partially purified human renin (IC50 values of 3.9, 1.6, and 1.4 nM, respectively). The ACDFOPA-based compounds 19c and 19f contain no natural amino acid fragments and have molecular weights which compare well with those of previously reported inhibitors of nanomolar in vitro potency. When administered intravenously to anesthetized, sodium-depleted marmosets at doses of 3 mg/kg, compounds 13a and 19c caused a marked reduction in mean arterial pressure, but in the same animal model at 30 mg/kg, oral activity was not seen.