In the rat lung, primary saccules are transformed into alveoli from postnatal Days 4 to 13, after which time there is a 20% reduction in the number of lung fibroblasts as the interstitial volume of the alveolar walls decreases. Our objective was to determine whether apoptosis is a factor in the observed decrease in the number of interstitial lung fibroblasts beyond Day 13. We used both histologic and flow cytometric assays to detect in lung fibroblasts the DNA fragmentation and condensation that are characteristic of apoptosis. In addition, we evaluated levels of bcl-2 and BAX messenger RNAs (mRNAs) using a reverse transcriptase-polymerase chain reaction (RT-PCR) assay. Apoptotic cells were quantitated in glycol methacrylate-embedded sections of neonatal rat lungs using the terminal transferase dUTP-digoxygenin nick end-labeling (TUNEL) method. Although TUNEL-positive interstitial cells were observed in the lungs of rats ranging in age from 10 to 16 d, a dramatic increase in apoptotic cells was seen on Day 17. Although diminished in number, TUNEL-positive cells were still present on Day 28. Hoechst-stained apoptotic bodies were observed in isolated lung cells that were vimentin-positive and factor VIII-negative, which identified the apoptotic cells as fibroblasts as opposed to endothelial cells. Flow cytometric analysis of freshly isolated lung fibroblasts stained with Hoechst 33342 indicated a 24% increase in chromatin condensation in cells from 17-d versus 16-d rats. DNA fragmentation was also quantitated by flow cytometry in freshly isolated fibroblasts labeled with BODIPY-conjugated dUTP in the presence of terminal deoxynucleotidyl transferase. The percentage of lung fibroblasts containing fragmented DNA was 51.4 +/- 13.4 in 17-d, 36.9 +/- 8.6 in 18-d, and 13.8 +/- 5.4 in 19-d rat pups. Finally, evaluation by RT-PCR indicated that on postnatal Day 17, mRNA for bcl-2, which inhibits apoptosis, was decreased to 73.5 +/- 11.4% (P < 0.001) of Day 5 controls; whereas mRNA for BAX, which enhances apoptosis, was increased to 243.0 +/- 102.0% (P < 0.001) of Day 5 values. These results demonstrate that rat lung fibroblasts undergo apoptosis after the completion of alveolarization, and suggest that this decrease in fibroblast number plays an important role in the thinning and remodeling of the alveolar walls of the lung.