Five gravity anomaly profiles across the Sea of the Okhotsk and the Kuril island arc and trench and two profiles from Fiji to Tonga are presented and studied. The four profiles across the Kuril arc are similar and differ from the profile extending off southern Kamchatka. The contributions to gravity anomalies from the crust and mantle are investigated by means of continuous seismic refraction measurements along tracks coincident with the gravity profiles. The mantle gravity anomalies computed from various density models are positive over the island arc, trench, and adjacent Pacific basin and amount to at least +50 mGal. Gravity anomalies due to the mantle suggest the existence of a denser slab 50--60 km thick parallel to the seismic zone. The positive gravity effect of the slab appears to be balanced over the Okhotsk basin by the negative effect of the heated, less dense mantle. The local minimum of Bouguer anomalies occurring over the shoreward flank of the Kuril trench is rather insensitive to the density contrast adopted. The sediments measured seismically account for this Bouguer minimum. The gravity field over the Kuril and Tonga ridges supports the idea of reduced density under the volcanic chain. The local minimum of gravity anomalies associated with the northwestern boundary of the Okhotsk basin is probably related to the fault separating the Academy of Sciences rise from the basin. The ocean-to-continent trend of the free-air anomalies in the Kuril area is unlike that in Tongo, which might be related to features of heat flow anomalies in the Kurils.