The pituitary hormones LH, FSH, and TSH are secreted as dimers of two subunits, a and β. The a-subunit, identical in all three hormones, is produced by the pituitary in excess of β and secreted as free subunit. Bovine free a does not combine with purified β-subunit and contains an extra O-linked oligosaccharide not found on dimer a. We have developed an assay to quantitate this modified form of a in the medium of incubated steer pituitary slices. The assay, based on reverse phase HPLC analysis of radiolabeled a-subunit tryptic peptides, shows that under basal conditions, 75% of secreted free a is 0-glycosylated. When the secretagogue LHRH is added to the slices, a 14-fold increase in LH dimer release is observed, but secretion of the modified a is increased by only 2-fold. Our results indicate that the majority of free a-subunit secreted by the pituitary contains O-linked oligosaccharide, and that secretion of this form of a differs from that of LH dimer.