The present study examines the influence of Ca2+ as (CaSO4), dissolved organic carbon (DOC) and pH on chronic water-borne lead (Pb) toxicity to the larval fathead minnow (Pimephales promelas) under flow-through conditions. The 30 day LC50 for low hardness basic test water (19 mg CaCO3 L(-1)) was 39 (range: 27-51) microg dissolved Pb L(-1) and was greatly increased by increasing concentrations of CaSO4 and DOC to as much as 1903 (range: 1812-1992) mug dissolved Pb L(-1). Both reduced and increased pH (6.7 and 8.1, respectively) compared to control pH of 7.4 appeared to increase Pb toxicity substantially. Whole body Pb accumulation did not reflect water chemistry and thus exhibited no correlation with Pb induced mortality. One possible explanation for this lack of correlation is that mortality occurred predominantly during the first 4-6 days of exposure, whereas Pb accumulation was determined in surviving fish at the end of 30 days of exposure. Chronic Pb exposure resulted in a general iono-regulatory disturbance affecting K+, Na+ and Ca2+ homeostasis. However, recovery of Na+ and K+ levels and reversal of effects on Ca2+ homeostasis during continued exposure strongly suggest fathead minnow can acclimate to Pb. The gills accumulate the highest Pb concentrations during chronic exposure but the skeleton contains the largest mass of Pb by contributing up to approximately 80% of whole body Pb. In conclusion, water chemistry characteristics like Ca2+ and DOC should be considered for chronic water quality criteria.