A time-intensity based method for photoacoustic blood flow measurements was proposed in last year's meeting. The method made use of the strong photoacoustic response of gold nanospheres and the "wash-out" characteristics of the nanospheres were analyzed. In this paper, we develop a new quantitative technique for measuring blood flows based on the "wash-in" characteristics of the nanoparticles. In particular, the technique makes use of the shape dependence of the optical absorption of gold nanorods (i.e., cylindrical nanoparticles) and the transitions in their shape induced by pulsed laser irradiation. The photon-induced shape transition of gold nanorods involves mainly a rod-to-sphere conversion and a shift in the peak optical absorption wavelength. The application of a series of laser pulses with the same laser energy will induce shape changes in gold nanorods as they flow through a region of interest, with quantitative flow information being derived from the photoacoustic signals from the irradiated gold nanorods measured as a function of time. To demonstrate the feasibility of the technique, an Nd:YAG laser operating at 1064 nm was used for irradiation and a ultrasonic transducer with a center frequency of 1 MHz was used for acoustic detection. Excellent agreement between the measured velocities and the actual velocities was demonstrated, with a linear regression correlation coefficient higher than 0.9. Compared to the wash-out analysis, the wash-in analysis is more suitable for measuring flows in microcirculation.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.