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Primary production (PP) in Lake Peipsi-Pihkva, the tripartite borderwaterbody between Estonia and Russia, was first measured in 1965-1966. Since 1970 there exists a continuous timeseries of monthly PP measurements from May to October. Detailed investigations of the seasonaland daily dynamics as well as the vertical distribution of PP were carried out in 1985-1987. The long-term average values of integral PP (PP{sub}int{end}) in Lakes Peipsi and Pihkva were equal (0.8 g C m{sup}-2{end} d{sup}-1{end}), although the values per cubic metre (PP{sub}max{end},) differed more than twofold and characterized L. Pihkva asa eutrophic lake and L. Peipsi as a transition type between meso- and eutrophic lakes. The years from 1973 to 1980, 1987 and 1991 were oflow productivity, while in 1971, 1983, 1988 and 1990 PP peaks occurred in both lakes. In the seasonal pattern PP{sub}int{end} had peaks in May and July. In June, after the spring bloom, PP as well as the chlorophyll a (Chl) and ATP content were low. The high Chl peak in autumn was probably built up by the degradation products of chlorophyll, as neither PP nor ATP increased. Seasonal changes in integral PP in L. peipsi could be well described R{sup}2{end} = 0.91) by an empiricalmodel relating PP{sub}int{end} to PP{sub}max{end}, Secchi depth (S) and total solar radiation (Q). In mixed conditions prevailing in bothlakes, PP was inhibited in the surface layer and its maximum was located at a depth of 0.25 ... 0.5 S. The threshold total solar radiation level for the onset of inhibition was between 1200 and 2000 kJ m{sup}-2{end} h{sup}-1{end} in May and July, and decreased to less than500 kJ m{sup}-2{end} h{sup}-1{end} in October. As a rule, inhibitionstarted in the morning at a higher irradiance than necessary for keeping it up during evening hours . When compared with PP{sub}max{end} , photosynthesis in the surface layer at noon was suppressed by 56 percent in May, by 45 percent in July and by 40 percent in October. [ABSTRACT FROM AUTHOR]