Geoid height increases in a step-like fashion across oceanic fracture zones owing to the juxtaposition of lithosphere with different ages and density structures. At a ridge/transform intersection, the initial geoid offset depends on the age difference across the fault and is approximately equal to 0.15 m/m.y. of age offset. The amplitude and shape of this anomaly will change along the length of a fracture zone depending on how the lithosphere cools. If the lithosphere thickens indefinitely (boundary layer model), then the geoid offset will remain constant although the width of the transition zone from one side of the fracture zone to the other will increase owing to both heat transfer across the fault and an increase in the average depth of compensation. If, however, the lithosphere eventually approaches a constant thickness (plate model), the total geoid offset will decrease with time. An analysis of 18 SEASAT altimetry profiles across the Mendocino fracture zone (offset 25--30 m.y.) indicates geoid anomaly amplitudes systematically decrease from more than 4 m across the eastern (younger) part of the fracture zone to about 1 m across the western (older) part of the fault. Although the geoid offsets across the older portion of the Mendocino fracture zone may have been under-estimated by as much as 20--25%, this large decrease in geoid offset strongly suggests that the lithosphere approaches a constant thickness along the older part of the fracture zone. A plate thickness of ~100 km fits the observed change in geoid offset along the Mendocino, but with uncertainties in the offset determination, the age difference across the older part of the fault, and the presence of areas of anomalous depths on both sides of the fracture zone, this probably represents a minimum estimate of the total plate thickness.