To visualize ASM contraction in vitro, we measured changes in cross-sectional area and inner circumference of isolated porcine and human bronchi in response to acetylcholine or carbamylcholine chloride (Carbachol) using high-frequency ultrasound. A mechanical ultrasonic catheter (20 MHz; diameter, 1.7 mm; echo element, 1 mm2) demonstrated three histologic layers (mucosa, cartilage and adventitia) and allowed measurement of dose-dependent contraction of porcine bronchi over time. Acetylcholine (10(-4)M) significantly decreased the cross-sectional area by 17 +/- 2.85 percent but did not change the circumference. The mean initial value for area was 0.6 +/- 0.08 cm2 and for circumference was 3.0 +/- 0.24 cm (n = 5). The EC50 of acetylcholine was 1.5 x 10(-8) M. Carbachol (10(-4)M)-induced contractions of human bronchi were also observed under the same conditions. In conclusion, high-frequency ultrasonic imaging can be used to study the morphology of ASM contractions in vitro, providing valuable information on the natural dynamics of ASM.