A high frequency ultrasound system for viscoelastic tissue property measurenment is being developed. A dual-element transducer was used for localized application of acoustic radiation force (ARF) pulses and monitoring of the induced shift. M-mode RF data was evaluated using a correlation based shift estimation algorithm . These displacements are used as input to an on-line tracking of the shear modulus and shear viscosity using an extended Kalman filter (EKF) which utilizes a second order constitutive model of viscoelastic response with unknown parameters. The algorithm was tested using tissue mimicking phantoms described by Nightingale et al , we also demonstrate the feasibility of evaluting the effect of drug in in-vitro porcine eye.