Rain events may impact the chemical pollution burden in rivers. Forty-four small streams in Germany were profiled during several rain events for the presence of 395 chemicals and five types of mixture effects in in vitro bioassays (cytotoxicity, activation of the estrogen, aryl hydrocarbon and peroxisome proliferator-activated receptors and oxidative stress response). While these streams were selected to cover a wide range of agricultural impacts, in addition to the expected pesticides, wastewater-derived chemicals and chemicals typical for street run-off were detected. The unexpectedly high estrogenic effects in many samples indicated impact by wastewater or overflow of combined sewer systems. The 128 water samples exhibited a high diversity of chemical and effect patterns, even for different rain events at the same site. The detected 290 chemicals explained only a small fraction (<8 %) of the measured effects. The experimental effects of designed mixtures of detected chemicals that were expected to dominate the mixture effects of detected chemicals were consistent with predictions for concentration addition by a factor of two for 94 % of the mixtures.Overall, the burden of chemicals and effects were much higher than previously detected in surface water during dry weather with the effects often exceeding effect-based trigger values. In-vitro bioassay test batteryEndocrine effects • ER Metabolism • AhR • PPARy Adaptive stress response: • Oxidative stress Chemical analysis Sampling triggered by rain events Mixture models

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