1 mg ovine prolactin was injected intramuscularly into rabbit fetuses (24th day of gestation) located in one of the two uterine horns exposed by laparotomy (n = 12). Fetuses in the other uterine horn were injected with an identical volume of vector and served as controls (n = 13). 2 days later the fetuses were removed by a second laparotomy and sacrificed. Analysis of lung tissue composition yielded the following results: (a) the prolactin-treated group of fetuses showed 40% higher total lung phospholipid content (17.0 +/- 0.8 micronmol/g) than the control group (12.2 +/- 0.5 micronmol/g); (b) the prolactin-treated group had a 67% higher lung lecithin content (8.7 +/- 0.8 micronmol/g) than the control group (5.2 +/- 0.4 micronmol/g); (c) dipalmitoyllecithin accounted for 67% of total lung lecithin in the prolactin-treated group and 44% in the control group. These differences were statistically highly significant (P less than 0.001). However, between the prolactin-treated and the control groups, there were no statistically significant differences in body weight and length, lung weight, the ratio of lung weight to body weight, DNA, protein and, water content. These results suggest that prolactin might be a trigger of lung surfactant synthesis in the rabbit fetus.