The thermosensitivity of the photosynthetic apparatus of 12-day-old seedlings of Pisum sativum cv. Ran was investigated within the temperature range of 2–50 °C. Fluorescence quenching was used as a criteria. The photochemical (q P ) and non-photochemical (q N ) fluorescence quenching were measured after a 5-min treatment at the respective temperature. It was established that low and high-temperatures strongly affect the pattern of the fluorescence induction curve. The results obtained show that q P increases with the temperature rise from 2 to 35 °C and at temperatures higher than 40 °C it decreases sharply. Temperatures enhancing from 2 to 25 °C led to a decrease of qN. The values of this parameter increased strongly at temperatures higher than 45 °C. The results showed that within the temperature range of 15–40 °C the photochemical quenching dominated over the non-photochemical one. The influence of temperature on the quantum efficiency of PS2 photochemistry ϕ PS2 was very similar to that of q P . It is concluded that the physiological state and the activity of PS2 are preserved in a wide temperature interval. The unfavourable temperatures provoke the switching on of the protective mechanisms as a qN enhancement and stimulation of cyclic electron transport through PS1, which allows the plants to endure the stress conditions.