A preliminary account of the histological effects of C14 on the leaves of 5 spp. of drug-producing plants, but primarily on Digitalis purpurea and D. lanata. The exptl. plants were kept in closed illuminated containers into which 100[mu] c units of C14 were introduced at intervals of 2-6 days over 28- to 33-day periods. CO2, nutrient solu., and water were supplied. Different plants were supplied 400, 900, or 1400[mu] c of C14O2; little difference in responses was shown. All dosages of C14, however, did produce alterations in the pattern of tissue differentiation in the leaves.