Water and bottom sediment samples from eight stations in the wetland of the delta of the rivers Axios. Loudias and Aliakmon were collected during 1992 and 1993 and analysed for pesticide residues. Eleven herbicides, atachlor, atrazine, 2,4-D, diuron, MCPA, metolachlor, metribuzin, molinate, prometryne, simazine and trifluralin and three organochlorine compounds alpha-BHC, lindane and 4,4'-DDE (DDT mebolite) were detected in estuaries of the Axios, Loudias and Aliakmon rivers as well and in the Thermaikos Gulf Highest concentrations of alachlor, atrazine, diuron, MCPA, metolachlor, metribuzin and prometryne in water samples were detected during the period of May to August. Six herbicides, alachlor, atrazine, metolachlor, molinate, simazine and trifluralin and three organochlorine compounds, alpha-BHC, lindane and 4,4'-DDE were found in significant amounts in estuarine sediments and in the Thermaikos Gulf. The amounts of each pesticide released were calculated using a collective model dependent on pesticide concentrations, river water flow rates and on their changes with time. Pesticides are released from the agricultural fields and transferred to the Thermaikos Gulf through the Axios, Loudias and Aliakmon rivers. The percentages of the total amount of pesticides released through the rivers to the Thermaikos gulf are estimated as 1.7 and 0.78% for alachlor, 2.5 and 1.0% for atrazine, 0.5 and 0.13% for 2,4-D, 1.1 and 0.1% for metolachlor, 7.4 and 2.5% for metribuzin, 0.8 and 0.25% for molinate, 4.5 and 4.4% for prometryne, 0.4 and 0.6% for simazine, 0.3 and 0.06% for trifluralin and 0.3 and 0.25% for lindane for the periods of 1992 and 1993, correspondingly. An estimation of the percentage of released amounts of diuron (2.3%) and MCPA (0.3%) was only possible for 1992.