Using an assay of the geotactic behavior of the fruit fly, we have quantified anesthetic effects in Drosophila melanogaster. We determined dose-responses for nine conventional anesthetics and a series of n-alkanes. For the conventional anesthetics, anesthetic potency and olive oil/gas partition coefficients are well correlated. However, as one ascends the series of alkanes of chain lengths 5-11, the increase in anesthetic potency is considerably less than that predicted by olive oil solubility. These examples of agreement and disagreement with the Meyer-Overton rule provide a basis for comparing the responses of fruit flies with those of higher organisms; such comparisons suggest that the anesthetic target is evolutionarily conserved. For alkanes with chain lengths greater than 11, anesthetic effects are variable and slow to develop. This behavior reflects limited volatility and provides no evidence for a cut-off in the action of alkanes at their target.