A 100 to 150% stimulation of Avena coleoptile segment elongation by the amino acid analogue p-fluorophenylalanine (FPA) has been observed. The effect is reversed by phenylalanine and is not seen with comparable concentrations of sodium fluoride. FPA does not alter elongation of red-irradiated segments. Stimulation by FPA occurs only when the apex is intact and the segments are incubated in the absence of exogenous auxin. In the presence of FPA, (14)C-leucine uptake by coleoptile segments is reduced by 34% and protein synthesis by 42%. When pre-incubated on labeled media and subsequently transferred to unlabeled media, segments fail to incorporate into the protein fraction any of the previously absorbed label. It is therefore difficult to ascertain whether FPA results in a genuine inhibition of protein synthesis in apical coleoptile segments. Possible mechanisms for the action of FPA and its relationship to light dependent elongation are considered.