Purulent otitis media was induced in 12 middle ears of Sprague-Dawley rats by inoculation of pneumococci type 3. The pressure required to open the eustachian tube was recorded 4 days later and was found to be 3.05 +/- 0.10 kPa (mean +/- SEM), which was 1.08 +/- 0.15 kPa lower than the mean value obtained in a control group of healthy rats (P less than or equal to .001). We also measured the effects of pulmonary surfactant instilled into the middle ear as well as isoprenaline injected intravenously. The presence of surfactant further reduced the pressure opening level by 0.72 +/- 0.05 kPa; this reduction did not differ from that found in the control group (P greater than .8). Isoprenaline reduced the pressure opening level in the control group by 0.20 +/- 0.03 kPa (P less than or equal to .001), and no reduction was found in the rats with otitis media. Although the part played by surface tension-lowering factors in eustachian tube function is still uncertain, such factors apparently also exert an effect in the inflamed tube. However, isoprenaline, which is considered to act by the secretion of surface tension-lowering substances, did not facilitate opening of the eustachian tube in rats with acute otitis media.