A major concern for drinking water companies are responsible for producing and supplying safe, high quality water for costumers. The drinking water quality should meet the standard at the customer tap, therefore the supplier is forced to analyze the water quality not only at the outlet from the treatment plant but in the distribution network as well. Drinking water quality depended on the size and complexity of distribution system and connected with water residence time (WRT) (Servais et al., 1992), and the network can be characterized as the biological and chemical reactor that interacts with the transported water (Gauthier et al., 1999). High level of natural organic matter (NOM) may facilitate the formation of disinfection by-products, increase the chlorine consumption. The biodegradable fraction of the NOM, biodegradable organic carbon (BDOC), leads to regrowth of microorganism and coliforms, including opportunistic pathogenic bacteria in drinking water distribution system (van der Kooij et al., 1982; Servais et al., 1992). This may increase the health risk for immunocompromised people. The aim of this study was to determine the concentration of total/dissolved organic carbon (TOC/DOC) and BDOC depending on WRT and biodegradation kinetics of organic carbon in water samples from different points of 1374 km distribution network made of cast iron pipes in Riga city, Latvia. The concentration of TOC ranged from 2.2 to 8.1 mg/l measured at different places (n=30) of the distribution network in Riga using a submersible, two beam spectrophotometer spectro::lyser™ (s::can Meßtechnik GmbH, Vienna, Austria) with on-line UV/Vis measurement. The concentration of BDOC ranged from 0.33 to 2.38 mg/l (n=25) measured using continuous bioreactor method with attached biomass (Ribas et al. 1991) and modified BDOC set-up developed within the EU project TECHNEAU with biofilter columns coupled in-series (Eikebrokk et al. 2007; Eikebrokk 2009). Results showed strong positive correlation between TOC and WRT in distribution network (Moment Correlation coefficient, r=0.81). WRT was determined using hydraulic model build up in Epanet 2.0 (Rossman, 2000) and in this study can be divided in three phases, which observed strong negative correlation with concentration of BDOC in distribution network for periods 2.82–13.72 h and 14.27–17.62 h (r=−0.72, n=8 and – 0.74, n=6 respectively) and no correlation between BDOC values and WRT>18.03 h (n=11). The biodegradation rates obtained from samples taken at different places of the distribution network in Riga ranged from 0.24×10-2 to 3.15×10-2 1/min (n=13) and there was a strong correlation with concentration of BDOC in these samples (r=0.78). The decrease of concentration of BDOC for the first hours (<24 h) within distribution network can be explained with consumption of substrate by bacteria. For more distant area in network (>24 h) broad range of BDOC values and increases of TOC can indicated on leaching process of NOM from deposits composed within distribution network. The results showed that dissolved organic matter is the important parameters and can be used as indicator to evaluate drinking water quality depending on the WRT in distribution network.