Synthesis and structure-activity relationship (SAR) studies of L-cysteine-based N-type calcium channel blockers are described. In the course of exploring SAR of the N- and C-terminal substituents, the L-cysteine derivative 4b was found to be a potent N-type calcium channel blocker with an IC50 value of 0.14 muM on IMR-32 assay. Compound 4b showed 12-fold selectivity for N-type over L-type calcium channels on AtT-20 assay. (C) 2002 Elsevier Science Ltd. All rights reserved.