Protein secretion through the endoplasmic reticulum and Golgi vesicular trafficking system is initiated by the binding of ADP-ribosylation factors (ARFs) to donor membranes, leading to recruitment of coatomer, bud formation, and eventual vesicle release. ARFs are ≈20-kDa GTPases that are active with bound GTP and inactive with GDP bound. Conversion of ARF-GDP to ARF-GTP is regulated by guanine nucleotide-exchange proteins. All known ARF guanine nucleotide-exchange proteins contain a Sec7 domain of ≈200 amino acids that includes the active site and fall into two classes that differ in molecular size and susceptibility to inhibition by the fungal metabolite brefeldin A (BFA). To determine the structural basis of BFA sensitivity, chimeric molecules were constructed by using sequences from the Sec7 domains of BFA-sensitive yeast Sec7 protein (ySec7d) and the insensitive human cytohesin-1 (C-1Sec7). Based on BFA inhibition of the activities of these molecules with recombinant yeast ARF2 as substrate, the Asp 965 –Met 975 sequence in ySec7d was shown to be responsible for BFA sensitivity. A C-1Sec7 mutant in which Ser 199 , Asn 204 , and Pro 209 were replaced with the corresponding ySec7d amino acids, Asp 965 , Gln 970 , and Met 975 , exhibited BFA sensitivity similar to that of recombinant ySec7d (rySec7d). Single replacement in C-1Sec7 of Ser 199 or Pro 209 resulted in partial inhibition by BFA, whereas replacement of Gln 970 in ySec7d with Asn (as found in C-1Sec7) had no effect. As predicted, the double C-1Sec7 mutant with S199D and P209M was BFA-sensitive, demonstrating that Asp 965 and Met 975 in ySec7d are major molecular determinants of BFA sensitivity.