The study was carried out in order to analyse the changes in photoplethysmographic (PPG) signal waveform, which are caused by cold and warm stimulation. The study was carried out on 7 subjects. The right hand was immersed in cold and warm water up to the wrist during the experiment. The PPG signal was registered from right hand index finger. As a reference, the temperatures of index finger and pulse transit time (PTT) values were measured. A correlation of r=-0.70 was found between normalised slope of the PPG signal and the subject's age. The normalised slope increased noticeably due to the cold and decreased due to the warm water stimulation. It was opposite to the expected results with PTTs and the behaviour has to be investigated in future studies.