The impact of human activities and environmental factors on the fluctuation of chemical and physicochemical parameters along the Pinios River and its tributaries was studied. Their seasonal variations throughout the years 1996-1998 are also presented. Most of the parameters (physical or chemical) measured in this survey exhibited high spatial and temporal variability. High temperatures during the warm period, attributed both to meteorological conditions and to the geographical relief of Thessalia plain, cause a restriction of the water flow, an accumulation of organic matter and the depletion of the dissolved oxygen in the water. Conductivity and hardness are high during the warm and wet period for different reasons. At the seaward part of the river high conductivity and hardness values indicate extended admixture of seawater. COD values fluctuated seasonally. Among the studied stations along the Pinios River the most polluted was the area where the river has passed the city of Larissa.