Export of labeled material derived by continuous photosynthesis in 14CO2 was monitored with a Geiger-Müller detector positioned next to an exporting leaf blade. Rate of export of labeled material was calculated from the difference between rates of retention and net photosynthesis of labeled carbon for the observed leaf. Given certain conditions, including nearly constant distribution of labeled material among minor veins and various types of cells, count rate data for the source leaf can be converted to rate of export of carbon. Changes in counting efficiency resulting from changes in leaf water status can be corrected for with data from a transducer which measures leaf thickness. Export data agreed with data obtained by monitoring the arrival of 14C in the sink region; isolated leaves gave values near zero for export. The method allows continuous nondestructive measurement of export of labeled carbon from a given leaf on an intact plant. The technique detects changes in export with a resolution of 10 to 20 minutes.