Fig. 2. Dose–response data for the enantiomers of isoflurane causing a loss of righting reflex in rats. (A ) S  (+)-isoflurane. The dose–response curve had values of ED⁵⁰= 23.3 ± 0.8 μl/kg and slope parameter h = 12.0 ± 3.2; 60 animals were used (in several cases the data points overlap). (Inset ) Gas chromatogram trace for S  (+)-isoflurane. Analysis showed the material to be 99.5% isoflurane, of which 100.0% was the S  (+)-enantiomer and 0.0% was the R  (−)-enantiomer. (B ) R  (−)-isoflurane. The dose–response curve had values of ED⁵⁰= 32.7 ± 1.5 μl/kg and slope parameter h = 11.9 ± 3.8; 40 animals were used. (Inset ) Gas chromatogram trace for R  (−)-isoflurane. Analysis showed the material to be 99.6% isoflurane, of which 99.4% was the R  (−)-enantiomer and 0.6% was the S  (+)-enantiomer.