For normal mouse fibroblasts four stages of their spreading in vitro are distinguished: the spherical cell stage; the stage of early appearance of processes, Lamellae formation; the stage of maximal cell spreading. Measurements of the duration of every stage were performed by means of time-lapse microcinematography under phase-contrast microscope. It is proved that the first and second stages are elementary ones of Puasson's kinetics and the following ones are complex stages of the same kinetic model. The mean time at these stages is respectively: 25+/-3.2, 12.5+/-1.5; 123+/-8; 162+/-25 minutes.