The objectives of these experiments were to identify inhibitors of RNA and protein synthesis that would be effective in preventing fiber development, and then to use these to determine critical periods of synthetic activity in early fiber development. The transcription inhibitors, amanitin and cordycepin, and the translation inhibitors, anisomycin and puromycin, were found to be effective in preventing fiber development Maximum ineffective low dose and minimum effective high dose were established for each. Synthesis of RN and protein in ovules growing in axenic culture was measured as incorporation of tritium labelled uridine or leucine, respectively, into TCA-insoluble product. Steady-state exchange of radiolabel in cultured ovule required approximately two days, although uridine incorporation was highly variable. The period of highest incorporation was between 2 and 6 days postanthesis. Both RNA and protein synthesis declined rapidly after 6 days. When ovules were cultured in the presence of inhibitor, then allowed to incorporate label led RNA or protein precursor at time intervals of 2 days thereafter,highly variable results were obtained. The results were confounded by the fact that incorporation into whole ovules, rather than fibers alone, ha to be measured. The general trend, as expected, was to decrease protein and RNA synthesis in the presence of inhibitors.