The distribution of PDI, ERp61 and ERp72, members of the protein disulfide isomerase (PDI) family of endoplasmic reticulum (ER) proteins, was determined in various murine tissues. Relative amounts of mRNA were measured using a quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. Protein levels were determined from Western blots. In most tissues, protein levels paralleled the amount of mRNA for each PDI family member. The tissue distribution of the PDI and ERp72 mRNAs was similar, although ERp72 was not as abundant as PDI in the tissues. The tissue distribution of the ERp61 mRNA was significantly different from the two other family members. To help define potential hormonal or maturational differences in the regulation of expression of PDI family members, mRNA was measured in the frontal cortex, liver, pituitary gland and uterus at timed intervals during postnatal maturation. Except in the pituitary gland, the mRNA levels at 10 days and 22 days after birth were essentially identical to those in the adult. The ERp61 and ERp72 mRNAs were present at 2- to 3-fold higher levels in the pituitary glands of the 10- and 22-day-old mice, than in the adult mice. In addition, the pituitary gland PDI mRNA was 2- to 3-fold higher in 10-day-old mice than in adults. In general, levels of PDI family members were higher in secretory tissues than in other tissues in both immature and adult mice.