Photographic Zenith Tube Zeiss (PZT) has been regularly used at Ondřejov Astronomical Observatory to determine latitude variations and clock corrections within the frame of Earth rotations service
since 1973. About 45 thousand star transits near the zenith of the observatory were observed and used to improve positions and proper motions of the stars three times, during the history of the observations. To derive the first two catalogues, PZT78 and PZT83 (based on the observations in 1973-1978 and 1973-1983 respectively), PZT observations were combined with star positions in AGK2 and AGK3 catalogues in order to obtain proper motions with higher precision. The most recent catalogue, PZT86, is based solely on PZT observations in the period 1973-1986. The algorithms used to derive all these catalogues as well as their precision and accuracy are discussed in the paper, and their comparison with AGK2/AGK3 given.