The Marmara Sea region is deforming alone, the branches of the North Anatolian Fault (NAF), which is the boundary between the Anatolian and the Eurasian plates. We evaluate Global Positioning System (GPS) measurements performed from 1992 to 1999, up to the 17 August 1999 Izmit earthquake, to quantify the interseismic strain accumulation pattern in the Marmara Sea region. We compute interseismic velocities in a frame fixed to the Eurasian plate at 136 GPS points. To compute the frame-independent strain rates, we first smooth the north-south and the east-west velocity components separately to obtain the values at grid nodes, and then we compute directional derivatives and strain rates. Shear-strain rates reveal that the Marmara Sea region is within the plate boundary deformation zone having a width of about I 10 kin. The largest shear-strain rate accumulation is along the northern branch of the NAF, with maximum shear-strain rate reaching 220 nstrain/yr in the Marmara Sea. We calculate clockwise rigid body rotation rates with a maximum of 10 deg/m.y. along the nor-them branch while the Anatolian plate rotates anticlockwise. Dilatation rates display adjacent local tensional and compressional areas in the east-west direction within the Marmara Sea. Assuming, the uniform simple shear, we compute the NAF slip rates vary between I I and 26 mm/yr, with a minimum around Izmit, and increase east and west of Izmit.