The paper deals with the study of PMT's afterpulse noise into the total lidar return signal. An attempt is de-scribed to determine the distribution function of PMT's afterpulses in laboratory conditions based on the use of PMT's pulse transient function approach. Using the data of lidar sensing of the atmosphere along vertical paths the distortions of lidar returns due to PMT's afterpulsing are obtained as functions of the sensing range and repetition frequency of sounding pulses. The effect of the afterpulses on the restored scattering ratio profile is revealed. Techniques for the instrumental and analytical corrections for PMT's afterpulsing noise are suggested.