Abstract. We present the results from observing the circumstellar maser emission of the M-type supergiant S Per in the 6 16 -5 23 water-vapour line at 1.35 cm. The observations were carried out in 1981-2002 (JD = 2 444 900-2 452 480) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences. The H 2 O spectra obtained represent an unprecedented long, uniform dataset on this star. We discuss the properties of the optical and maser variations of S Per, together with particulars of the available VLBI maps. The close relation between maser and optical variations favors a model in which mass-loss is episodic. Changes observed in the total H 2 O line flux follow the visual light curve with a delay of 0.01 to 0.5P, where P ≈ 800 d is the mean light cycle for S Per. The feature at V LSR = −44 km s −1 flared in July 1988, which seemed to be the response of the maser to an unusually bright optical maximum. The position of the −44-km s −1 feature on the VLBI maps coincides with the direction toward the optical stellar disc, which can be explained by amplification of enhanced stellar continuum by the H 2 O line.