Ultrasound is known to produce a range of non-lethal responses in cells and tissues. Frequencies in the kilohertz ultrasound range have been shown to produce relaxation in large arteries (Fischell et al., 1991; Steffen et al., 1994). The present work explores the effects of insonation at MHz frequencies, representative of those used diagnostically and therapeutically, in an in vitro preparation of the carotid artery. Fresh 1cm wide rings of equine and bovine common carotid artery obtained from the abattoir were mounted in a purpose-made myograph. They were immersed in a bath of Krebs-Ringer buffer at 37 °C and were positioned at the focus of an ultrasound beam from a weakly focused 3.2 MHz source. Continuous wave insonation produced a contraction, the tension increase over the first 2 minutes was rapid; this was followed by a slower increase for the duration of the exposure up to 15 minutes. At a power of 145 mW contractile forces of 1.1 ± 0.09 mN (mean ± sd, n = 100) were measured, this was approximately 10% of the maximum force generated by noradrenaline (1mM). The magnitude of the response was weakly dependent on power in the range 72-145 mW and was not significantly different for pulsed and continuous wave stimulation where time averaged power was constant. The effect was unaffected by mechanical removal of the endothelium. The ultrasound beam generated insufficient acoustic pressure to produce a measurable effect and streaming at the vessel surface was very small compared to flow rates known to produce physiological effects. The temperature rise at the beam focus was approximately 0.3 °C and we hypothesise that this contributes to the observed response, probably through changes in ion channel activity in smooth muscle cell membranes (Mustafa and Thulesius, 2005).