Using a novel fluorescent probe for Pi, a method for the direct visualization of Pi release from reactivated flagellar dynein ATPase has been developed. The probe undergoes a fluorescence increase when it binds Pi. The technique involves simultaneous imaging of demembranated sperm tails by epi-fluorescence and dark-field microscopy, and the use of the caged ATP technique for axoneme reactivation. To limit diffusion and thus maintain the released Pi within the observed field of view, the assay is carried out within a minute droplet under oil (volume 5-15 pl). The video output of a recursively filtered ICCD camera is used to visualize the fluorescence signal, which is subsequently digitized and automatically analysed on a PC. A major advantage of this technique is that it enables simultaneous analysis of the ATP-utilization rate and the motility of the reactivated axonemes.