In this study washed, 20-35 aged human platelets were used. The effect of insulin, by itself and with thrombin activitation, on the depolarisation, aggregation and lysosomal enyzmes secretion was investigated. On the other hand, it was seen that insulin effected thrombin activated depolarisation and aggregation. It was figured that the increase in depolarisation becomes possible ony after an ‘insulin treshold’ has been reached. It was seen that the insulin, dose dependently, effects first the mechanism related with depolarisation and with higher doses the mechanism related with aggregation. It was further noticed that at doses studied that are responsible for an increase in depolarisation and aggregation, insulin had no effect on lysosomal enyzmes secretion dependent on thrombin activation. Keywords :