Left pneumonectomy (PX) was performed on 14-day-old pregnant rats. Serum growth hormone (GH), lung somatomedin-C-like immunoreactivity (SmCJ, and lung bomhesin-like immunoreactivity (BLI), using optimized radioimmunoassays and lung protein concentration (P), were measured 3 h, and 1, 2, 3, 5, and 7 days following pneumonectomy. These levels were compared to two groups of similar animals: sham operated animals and animals not subjected to surgery. Serum GH, lung SmC, and BLI levels were similar in the last two groups of animals, suggesting that surgery had no effect on GH, SmC, and BLI levels. These two control groups were combined and compared to the post-pneumonectomy animals. The post-pneumonectomy animals had significantly higher levels of serum GH at postoperative day 3 and significantly higher levels of SmC at days 2 and 5 without any significant difference in total BLI level and body weight. These results suggest that, first, GH and SmC may play a part in post-pneumonectomy compensatory lung growth and these two may also be interrelated in this response and, second, BLI material(s) perhaps do not play a role in post-pneumonectomy lung growth.