Since March 1994, the Institut de Tecniques Energetiques (INTE) of the Universitat Politecnica de Catalunya (UPC), has been measuring environmental gamma dose rate and the airborne radon concentration levels outdoors, at the ESCRA Station, which is located in Barcelona. ESCRA simultaneously measures several meteorological parameters as well. As it is well known, the gamma dose rate suffers noticeable increments during rain intervals, which in our case range from10% up to 40%, mainly caused by the deposition of the radon daughters ²¹⁴Bi and ²¹⁴Pb. These increments do not seem to be correlated with rainrate, rain interval duration, precipitation volume, or other meteorological parameters. In order to develop models that explain the observed dose enhancement, it is interesting to determine the cencentration of the gamma emitters in rainwater and, due to the relative short half life of the two nuclides mentioned above, this must be done as it rains. To this end, we have developed an on-line gamma spectroscopy system for rainwater. The operation of this device, which has been automated by means of a Lab View program, consists of the following steps: first, 250 cm³ of water is collected and, if the rainrate is high enough, is transferred to the measurement tank, a Marinelly-like container located underneath a HPGe solid state detector. All the system is surrounded by a lead shield. Preliminary measurements carried out with this device yielded radon daughter concentrations ranging from 50 Bq/l up to 1600 Bq/l.