To introduce a methyl group at C-16 instead of C-13, dehydrocostus lactone was allowed to react with an ethereal solution of diazoethane. To obtain structure-biological activity data, thirteen new pyrazoline derivatives from eudesmanolides and guaianolides were synthesised. The carbon chain at C-13 was extended by treating a 13-methyl derivative of dehydrocostus lactone with diazomethane and diazoethane to form substituted pyrazolines. The same sequence of reactions was performed on alantolactone, isoalantolactone and isodehydrocostus lactone, which formed substituted pyrazolines. The structure of each compound was elucidated by spectroscopic techniques including IR, 1H NMR and13C NMR spectra. All thirteen generated compounds were subjected to biological evaluation as plant growth regulators. The results demonstrate an overall increase in effectiveness compared to the parent compounds.