While substantial knowledge on the occurrence of pharmaceuticals in the environment is available, their behavior and fate in surface waters is still poorly understood. Therefore, the aims of this study were to analyze the short-term dynamics of selected pharmaceuticals along a 13.6 km long river stretch downstream of a wastewater treatment plant (WWTP), and to quantify their attenuation by a mass balance approach. Four acidic pharmaceuticals (bezafibrate, clofibric acid, diclofenac, naproxen) with different attenuation properties were measured over a period of three weeks at high temporal resolution, and in situ photolysis experiments were carried out. The average concentrations of pharmaceuticals were between 9 +/- 4 and 339 +/- 133 ng L(-1), corresponding to loads between 1.9 +/- 1.2 and 63 +/- 37 g d(-1) (n = 134). The temporal dynamics of pharmaceuticals was closely related to discharge of the WWTP and precipitation, and highest concentrations were observed at the beginning of a discharge event. During a dry period, naproxen was eliminated along the river stretch with a dissipation time (DT(50)) of 3.6 +/- 2.1 days while the other compounds did not exhibit significant attenuation. As photolysis and other abiotic processes were of limited quantitative relevance, the attenuation of naproxen can most likely be attributed to biotransformation.