Background: We previously showed that triiodothyronine (T3) stimulates muscle phosphoglycerate mutase (PGAM) activity and isozyme transition in rat skeletal and cardiac muscles. Methods: The effects of T3 on PGAM types B and M subunit expression in rat muscle during development are reported. Results: T3 administration during the first 21 days of rat life more than doubles type M PGAM mRNA levels, but produces minor effects on type B PGAM mRNA levels. The antihormone propylthiouracil (PTU) slightly decreases both type B and M mRNA levels, but this decrease is not statistically significant. Conclusion: Thyroid hormone influences PGAM mRNA isozyme levels differently and increases type M mRNA.