We labelled endocytic veiscles of living Mouse Fibroblasts Clone 1D, by incubating the cells in their culture medium containing Rhodamine‐Dextranes (RD). Microfluorimetric measurements showed that RD was internalized by pinocytosis. FRAP curves determined at 17 °C on cells incubated with RD showed that there were three classes of vesicles: quick moving vesicles, slow moving vesicles and immobile vesicles. The times t1and t2of fluorescence half recovery of the two mobile vesicule classes were proportional to the laser beam radius. The FRAP curves were considered as resulting from the flow of vesicles whose velocities were distributed according to a bimodal gaussian function. The average velocities depended on the incubation time and varied between 6 to 11 &mgr;m sec−1for V¯1and 0.3 to 0.7 &mgr;m sec−1for V¯2. This last velocity was ascribed to the saltatory motion of lysosomes and endosomes while V¯1was considered to represent the velocity of small vesicles which have not been detected so far by optical microscopy.