When labelled uracil was supplied to the solution bathing the cambial surfaces of bark strips of willow, uptake into the sieve elements took place as evidenced by the appearance of radioactivity in the honeydew of aphids feeding on the strips. When maleic hydrazide (MH) was also applied an inhibition of uracil uptake was observed. However uracil seemed to have no effect on MH uptake. The type of inhibition described above showed similarities to classical competitive inhibitors of enzymes.