Trends with spawning time in the ovarian weight of the Baltic herring ( Clupea harengus membras ) were studied from samples collected over the spawning season in 1988–1991, 1997 and 1998 in the Archipelago Sea, northern Baltic. Usually, ovarian weight was related to the size composition of the spawning shoals but there was also variation unrelated with female length. In the latter cases, ovarian weight first decreased in the beginning of the spawning season with decreasing fish length but it then increased again in the middle of the season, irrespective of length. In such years, the piecewise regression analysis suggested a breakpoint for the ovarian weight between the dates 14 June and 30 June. At the later part of the spawning season, ovarian weight was associated with temperature conditions and onset of zooplankton production, which suggests that environmental factors modify the reproductive properties of the herring in the Baltic Sea.