Background: In vivo study of bladder function has been limited to measurements of bladder pressure and intravesical volume without the capability for the assessment of the changes in bladder shape or size which ultimately reflects detrusor contractility and compliance.Aim: This study describes a new technique for in vivo assessment of bladder function using ultrasonic crystals placed in the bladder serosa to study bladder wall motion during both filling and voiding.Methods: Yearling sheep underwent laparotomy with placement of intravesical filling and pressure catheters, a transurethral flow probe and three small ultrasonic crystals (3 mm). One crystal was attached to the midportion of the bladder ventrally and two dorsally. Bladder pressure (mmHg), diameter (cm), urine volume (cc) and flow (cc/sec) were continuously recorded during both filling and voiding with a custom designed computer program.Results: The ultrasonic crystals demonstrated a linear relationship between bladder diameter and volume (a). Data obtained during voiding generated distinctive relationships between bladder diameter and pressure (b), and diameter and urine flow, each of which was comprised of at least two phases. Reduction in bladder size during voiding is associated with an increase in bladder pressure and a gradual reduction of urine flow which represents the detrusor smooth muscle contraction.Discussion: Ultrasonic crystals allow detailed study of bladder size, which combined with existing techniques, provides a new mechanism for study of bladder compliance and contractility.