On a number of occasions between 1970 and 1984, measurements of mass balance, surface velocity, and surface elevation were made along a 10.2 km flow line extending from the divide to the margin on Barnes Ice Cap, Baffin Island, Canada. The average mass balance rate on this Arctic ice cap appears to be controlled principally by summer temperature. Years of negative mass balance dominated during the 14 year period of the study; according to our measurements, this should have resulted in a thinning of an average of 2.65 m along the flow line. Surface-elevation measurements, however, suggest that the glacier thinned only ~ 1.7 m during this time period. The slightly compressive mean transverse strain can account for ~ 0.20 m of the discrepancy between these two figures. The rest is attributed to errors in mass balance. In particular, increases in density of old firn and freezing of water in crevasses could not be evaluated adequately from our measurements.The thinning was not uniform. Near the margin the glacier thinned ~ 5 m and near the divide, ~ 2.5 m, but between 3.5 and 5.5 km from the divide there seems to have been no substantial change in thickness. Thus, along the down-glacier half of the flow line the decrease in thickness was accompanied by an increase in slope.Within 4 km of the margin the decrease in thickness resulted in a decrease in horizontal velocity of up to ~ 15%, despite the increase in slope. Higher on the glacier, however, the horizontal velocity increased ~ 5%. These changes are broadly consistent with those estimated from the changes in shear stress calculated using the hydrostatic approximation.