Abstract Although intrapituitary conversion of T4 to T3 has been shown in rat pituitary homogenates, this deiodination has not been demonstrated in non-thyrotropic pituitary cells. GH3 cells, which produce growth hormone and prolactin, were used to demonstrate formation of T3 from T4 in monolayer culture. During a 1 hr incubation, the amount of T3 generated increased with increasing T4 substrate concentrations, reaching a plateau after 0.9 μM T 4. When dithiothreitol was added to the medium, production of T3 doubled at all concentrations of T4. T3 production in the presence of 2 μM T4 increased from .23 nM after 0.5 hr incubation to .8 nM after 3 hr incubation. Deiodination by this nonthyrotropic rat pituitary tumor cell is markedly enhanced by a thiol-protective agent.