Surfactant protein B (SP-B) is a constituent surfactant protein critical for normal lung function. Monomeric SP-B(1-25) (mSP-B(1-25)), a peptide based on the N-terminal domain of human SP-B, mixed in phospholipids partially restores lung function in surfactant-deficient animals. Because native SP-B is a homodimer, we synthesized and tested dimeric SP-B(1-25) (dSP-B(1-25)). Circular dichroism (CD) and Fourier Transform Infrared (FTIR) spectroscopy indicated that the secondary conformation of SP-B(1-25) was not significantly perturbed by dimerization. The effects on lung function were compared to phospholipids and Survanta in models of neonatal respiratory distress syndrome (RDS) and acute RDS (ARDS). Preterm rabbits born at 27 days of gestation received 100 mg surfactant / kg at birth and were ventilated for 1 hour with a tidal volume of 10 mL / kg. Dynamic compliance was monitored every 15 minutes and postmortem pressure-volume curves were measured. Adult rats were lavaged to induce surfactant deficiency, treated with 100 mg surfactant / kg, and ventilated for 2 hours. Lung function was assessed using arterial blood gases and dynamic compliance every 15 minutes and postmortem pressure-volume curves. Lung volumes in both models and oxygenation in the lavaged rats were consistently higher in the dSP-B(1-25) than in the Survanta and mSP-B(1-25) surfactant groups. The data suggest that dSP-B(1-25) is more efficient in restoring lung function in neonatal RDS and ARDS than mSP-B(1-25) surfactant.