This study investigates the karst drainage network of Ogof Agen Allwedd, an extensive cave system underlying the plateau of Mynydd Llangattwg, South Wales. Water samples were collected between July and October 2002 from the peat moorland above the cave, drips within the cave, the Entrance Series, Main Streamway and its tributaries, and the resurgences in Clydach Gorge. Conductivity, temperature, pH, alkalinity, Ca, Mg, Mn and Fe were measured to characterize components of the hydrological system. Hydrochemical data for some of the inlets, such as Cascade Passage and Meander Passage, suggested rapid response times to recharge events, which may have important implications for flooding and pollution events in the cave. The response of the cave system to rainfall was further investigated by measuring relative changes in water-level and temperature at a major confluence in the Cascade Passage area on an hourly basis between September and November 2002. Response to rainfall was more complex than suggested by the hydrochemical data. The vadose drainage system can be described as a three-component system with (1) rapid conduit flow, (2) soil/fissure (epikarst) network flow, moderated by surface air temperatures and (3) a diffuse, percolation flow that is in equilibrium with the rock matrix temperatures at depth.