The amino acid sequence of the porcine beta subunit has been established by studies of peptides isolated after tryptic, thermolytic and staphylococcal protease treatments of the reduced and carboxymethylated chain. The primary structure of the amino-terminal region of the molecule has been solved by automatic sequencing of the reduced and tritium-labeled carboxymethylated subunit. The amino acid sequence of porcine follitropin beta subunit differs from that of its human counterpart by several amino acid replacements, deletion or addition of one or several residues. The porcine chain appears shorter at both its amino and carboxy-terminal ends. The chemical evolution of follitropin is briefly considered and compared to these of thyrotropin and lutropin.