Aquatic ecosystems could be enriched with inorganic and organic matters after road runoff discharge. In this context, we studied the effects of road runoff on river biofilms (periphyton). To operate under controlled conditions the experiments were carried out in two indoor. experimental streams. Glass slides were used as artificial substrates for the periphyton colonisation in the experimental streams. Current velocity was maintained at 12 cm s(-1) (slow) in the first experimental stream and at 50 cm s(-1) (fast) in the second one. The periphyton communities were periodically exposed to road runoff over 30 days. For this experiment, the road runoff was collected in settling basins of a motorway during rainfall events. The runoff was then characterised according to physical and chemical parameters. We tested two exposure durations (1 and 4 h) and two runoff dilutions (10% and 50%). Two laboratory experiments carried out during this study revealed that the biomass (AFDW: 0.92-2.83 g m(-2)), the chlorophyll a content (6.8-78.9 mg m(-2)) and the metabolic activity (net primary production: 61-334 Mg O-2 m(-2) h(-1) and respiration: 17.2-68.3 Mg O-2 m(-2) h(-1)) of the biofilms increased (p < 0.05). However, this was a function of current velocity, the runoff exposure duration, and the content in organic and inorganic elements present in the tested rainwater. Experimental streams constituted a simplified natural system, which did not allow the reproduction of all the environmental conditions. Thus, these experiments should be performed on natural sites. (c) 2005 Elsevier B.V. All rights reserved.