In vivo measurements of the viscoelastic properties of breast tissue are described. Ultrasonic echo frames were recorded from volunteers at while applying a uniaxial compressive force within a ramp time and holding the force constant for up to . A time series of strain images was formed from the echo data, spatially averaged viscous creep curves were computed, and viscoelastic strain parameters were estimated by fitting creep curves to a second-order Voigt model. The useful strain bandwidth from this quasi-static ramp stimulus was . The stress-strain curves for normal glandular tissues are linear when the surface force applied is between 2 and . In this range, the creep response was characteristic of biphasic viscoelastic polymers, settling to a constant strain (arrheodictic) after . The average model-based retardance time constants for the viscoelastic response were and . Also, the viscoelastic strain amplitude was approximately equal to that of the elastic strain. Above of applied force, however, the response of glandular tissue became increasingly nonlinear and rheodictic, i.e., tissue creep never reached a plateau. Contrasting in vivo breast measurements with those in gelatin hydrogels, preliminary ideas regarding the mechanisms for viscoelastic contrast are emerging.