Novel, transient prodrug forms of biologically active agents containing mercapto groups have (i) the structural formula (I): ##STR1## wherein X is O, S, or NR.sup.5 ;R.sup.1 S is the residue of any biologically active agent R.sup.1 SH;R.sup.2 is selected from the group consisting of straight or branched chain alkyl having from 1 to 20 carbon atoms; aryl having from 6 to 10 carbon atoms; cycloalkyl having from 3 to 8 carbon atoms; alkenyl having from 2 to 20 carbon atoms; cycloalkenyl having from 4 to 8 carbon atoms; alkynyl having from 2 to 20 carbon atoms; aralkyl, alkaryl, aralkenyl, aralkynyl, alkenylaryl, alkynylaryl, loweralkanoyloxyalkyl, carboxyalkyl, and lower alkanoyloxyalkyl wherein alkyl, aryl, alkenyl and alkynyl are as defined above; saturated or unsaturated monoheterocyclic or polyheterocyclic, or fused heterocyclic, either directly bonded to the carbonyl function or linked thereto via an alkylene bridge, containing from 1 to 3 of any one or more of the heteroatoms N, S or O in each heterocyclic ring thereof and each such ring being from 3- to 8- membered; and mono- or polysubstituted derivatives of the above, each of said substituents being selected from the group consisting of lower akyl, lower alkoxy, lower acyl, lower acyloxy, halo, haloloweralkyl, cyano, carbethoxy, loweralkylthio, amino, nitro, loweralkylamino, diloweralkylamino, carboxyl, carbamyl, loweralkylcarbamyl, diloweralkylcarbamyl and ##STR2## wherein R.sup.4 is hydrogen or alkyl having from 1 to 10 carbons;R.sup.3 is hydrogen, R.sup.2, lower acyl, cyano, haloloweralkyl, carbamyl, loweralkylcarbamyl, diloweralkylcarbamyl, --CH.sub.2 ONO.sub.2 and --CH.sub.2 OCOR.sup.2 ;R.sup.5 is hydrogen or lower alkyl; and further whereinR.sup.2 and R.sup.3 may be taken together to form a cyclizing moiety selected from the group consisting of ##STR3## with the proviso that when R.sup.1 S is the residue of a sulfur containing amino acid, then X cannot be NR.sup.5 ;(ii) the structural formula (I) wherein ##STR4## is the residue of any naturally occurring protein amino acid, the residue of any N-substituted naturally occurring amino acid, which N-substituent is lower alkyl or any amino acid protective group cleavable via hydrogenolysis or hydrolysis, or the residue of an N,N-lower dialkyl of C.sub.4 -C.sub.7 cycloalkylamino acid; and(iii) the non-toxic, pharmaceutically acceptable salts thereof.