Maternal diabetes during pregnancy is associated with increased risk of neonatal respiratory distress syndrome (RDS). Previous studies using rat models for the diabetic pregnancy have documented decreased amounts of surfactant protein mRNA in the lungs of fetuses. In this study, we measured fetal lung surfactant-associated protein A (SP-A) mRNA from diabetic rats treated with insulin by daily injection or osmotic pump. Lungs were taken from fetuses on gestational d 20, and RNA was isolated and subjected to Northern blotting and densitometry to quantify SP-A mRNA. Fetal lung SP-A mRNA from untreated diabetic pregnancies was 34 +/- 2.9% of control. Insulin treatment increased levels to 55 +/- 4.2% of control values. Fetal lung SP-A mRNA levels were affected by the timing, length, and effectiveness of insulin treatment. Although levels from all treatment groups were still less than control values, insulin treatment during the last 5 or 10 d of pregnancy resulted in a substantial increase in SP-A mRNA levels over those of from untreated diabetic pregnancies. However, fetuses from the group with insulin treatment for the entire pregnancy showed decreases in fetal SP-A mRNA levels. Although the mechanism(s) responsible for the effects of diabetes and its treatment on fetal SP-A expression remain unclear, it appears unlikely that hyperglycemia is the principal cause.