A tetrapolar 75-kHz, 0.2-mA constant-current electrical impedance measuring system was used to monitor urinary volume change over 12 four-hour sessions in 20 male and 20 female normal human subjects. Two spot voltage electrodes (E) and two spot current electrodes (I) were applied 5 cm above the symphysis pubis at 15 cm interelectrode distances, 7.5 cm bilaterally from the midline. Five measurements of impedance and skin temperature were made at 15-minute intervals over 4 hours. Specific gravity, impedance change, and volume were recorded with each voided specimen. Suprailiac and infrascapular skinfolds, and circumference at iliac crests were measured. The subjects drank 175 ml of fluids per hour during the testing session. Subjects remained in the supine position during measurements. They walked to the lavatory to void. Bladder fullness is defined as the urge to void. Results include: (1) a poor negative correlation between specific gravity of urine and impedance (r2 = 0.1240, p less than or equal to 0.01); (2) baseline impedance was dependent upon individual subject characteristics: in males skin area, skinfold thickness, and suprailiac circumference; in females skinfold thicknesses and time since last menstrual period; (3) impedance decreased with urinary bladder filling and increased upon voiding (p less than or equal to 10(-10)); (4) the cumulative sum test (CUSUM) predicted time to void in 78.9% of voids (p less than or equal to 0.05) and no void (when voiding did not occur) in 66.8% of no voids (p less than or equal to 0.05). Overall accuracy of the CUSUM test was 74.6% (p less than or equal to 0.05).