A balloon-borne instrument to measure total odd nitrogen (NOy) has been developed. A converter which enables catalytic conversion of NOy into nitric oxide on a heated gold surface is combined with a chemiluminescence detector. The conversion efficiency for nitrogen dioxide was measured to be close to 100% at pressures between 60 and 7 mb. The major source of errors in the balloon-borne measurements are the uncertainties in the estimates of the sample flow rate and the zero level of the instrument. The NOy concentration was measured at altitudes between 12 and 28 km with a precision of about 25% on a balloon experiment conducted at latitude 44 ¿N in June, 1989. The NOy concentration has been measured to be 1.5¿0.4, 3¿0.7, 10¿3, and 14¿4 ppbv at altitudes of 17, 20, 25, and 28 km, respectively. This was the first in situ NOy measurement in the stratosphere above 20 km.