An improved five-step procedure has been developed for the synthesis of the gibberellin plant growth inhibitor 16,17-dihydro-GA5 from gibberellic acid. The 3-13C and 3-14C derivatives of this material have also been prepared by excising C-3 from the carbon skeleton (ozonolysis of a Δ2-alkene derivative and subsequent retro-Claisen reaction) and then reassembling the A-ring by means of an intramolecular aldol reaction.Key words: gibberellin, growth inhibitor, degradation, Radio labelling, Wittig reaction.