The amino acid sequence of the α subunit of equine chorionic gonadotropin (eCG, also pregnant mare serum gonadotropin, PMSG) has been determined. Overlapping peptides from tryptic and chymotrypic digests were isolated by a two-dimensional peptide mapping technique and sequenced by the Edman procedure. The proposed amino acid sequence of eCG α is: (**Denotes carbohydrate attachment points.) This sequence differs significantly from that proposed by Rathnamet al. (1978) for equine follitropin α subunit; in particular, their sequence lacked the first fourteen residues. For the β subunit we have placed in sequence 104 amino acid residues by direct sequence determination and peptide overlap procedures; in addition, 37 residues have been placed provisionally by homology with the human chorionic gonadotropin (hCG) sequence and composition and/or sequence data for the peptides isolated in the present studies. Difficulties in the procurement of the hormone have stalled completion of the β-subunit amino acid sequence determination. The data now available indicate that eCG β-subunit is highly homologous to hCG β subunit and the β subunits of luteinizing hormone from the pituitary gland of the several species so far described. The proposed partial sequence of eCG β is: