Lung RE concentration after a single treatment or multiple treatments with VA alone, all- trans -retinoic acid (RA) alone, or combination of VA and 10% RA (VARA), Am580 alone, and combination of VA and 10% Am580 (VAAm). Neonatal rats were treated with a single dose, prepared and delivered as in Experimental Methods, which was given either early during the period of lung septation (PD4) (A), or near the end of the period of lung septation (PD14) (B), or as multiple doses on PD 4, 7, 11, and 14 (C). Lung total retinol concentration, which is mainly RE (see Materials and methods), was determined on tissue collected 6 h after the single dose or final treatment in the multiple dosing design. Data are presented as means ± SEM. Each treatment group included n = 4-6 neonates. Statistical analysis was conducted on log10-transformed data using one-factor ANOVA and Fisher’s Least Significant Difference posthoc test. Groups with different letters differed significantly, P < 0.05. Asterisks in panel C mark significant differences between the PD14 single dosing study (B) and multiple dosing study (C), tested for each treatment by unpaired t-test.