Twenty-three analogs of the ACTH-(4-10)-heptapeptide sequence, which forms the "active core" of adrenocorticotropin (ACTH) and related hormones, have been synthesized by the solid-phase method. These analogs all contain structural modifications at or near the 5-glutamic acid residue of ACTH. The peptides were purified to electrophoretic and chromatographic homogeneity. The peptides were assayed for lipolytic activity in an isolated cell system derived from rabbit adipose tissue. In this system, it was determined that residue 5 plays a very important "spacer" role in the peptide, but that this spacer function is not very dependent on the nature of the side chain of the position 5 amino acid. It was found, however, that a number of analogs containing basic residues (arginine or lysine) in position 3 and/or position 5 of ACTH-(3-10) and ACTH-(4-10) fragments have 5 to 10 times the activity of the respective parent peptides. The presence of a latent anionic locus in the rabbit fat-cell receptor for ACTH is suggested by this study.