The first half of this dissertation describes the synthesis and biological activities of a series of amino aldehydes; which were derivatives of the basic amino acids, arginine, lysine and ornithine. The synthesis of the amino aldehydes was complicated by the difficulty of producing an intermediate oxidation state (the aldehyde) in the presence of two other functional groups (the α-amino, and the side chain functionality). The amino aldehydes were of biological interest due to the fact that they were inhibitors of the proteolytic enzymes called kallikreins. The kallikreins are known to be involved with the renin-angiotensin system, arginine vasopressin, and the prostaglandins, in the regulation of blood pressure. The aldehydes were assayed for their ability to inhibit the kallikrein-mediated production of kinins, and by the inhibition of the cleavage of Nᵅ-tosyl arginine methyl ester (TAME) to the carboxylic acid. Two of the amino aldehydes (Nᵅ-t-Boc-Nᴳ-nitro-L-argininal and Nᵅ-t-Boc-Nᴳ-tosyl-L-argininal) were effective inhibitors in both bioassays at micromolar concentrations. The second part of the dissertation details the development of two syntheses of β-substituted analogues of cysteine. The first method was based on sulfenylation of Nᵅ-formyl-α, β-dehydro amino acid esters, followed by protection of the sulfhydryl group as the benzyl or para-methylbenzyl thioether. The Nᵅ-formyl and ester groups were cleaved by acidic hydrolysis, and the amino group was then blocked as the t-butyloxycarbonyl derivative. This procedure gave cysteine analogues which were suitable for direct use in solid phase peptide synthesis. A second, more efficient preparation of the cysteine analogues was based on the conjugate addition of lithium benzylthiolate (or lithium para-methylbenzylthiolate) to the Nᵅ-formyl-α, β-dehydroamino acid esters. This synthesis was more efficient since the cysteine analogues were generated directly in S-protected form. The fully protected intermediates were deprotected at the amino and carboxyl groups, followed by treatment with di-tert-butyl dicarbonate. The Nᵅ- t-Boc-β-S-benzyl cysteine analogues (or Nᵅ -t-Boc-β-S-para-methylbenzyl) also were suitable for direct use in solid phase peptide synthesis.