The climax of "Dr. Heidegger's Experiment" occurs when Mr. Medbourne, Mr. Gascoigne, and Colonel Killigrew, in their tussle over the Widow Wycherly, overturn the table on which the vase of water is resting. The vase shatters into a thousand fragments, and the water flows across the floor, renewing the youth of a dying butterfly as it does so.

The rising and falling action are delineated with particular clarity in this story, as the rising action consists of the four old people becoming young (or imagining that they are); then, after a literally shattering peak of the story, they return to their former, aged selves without having learned anything. The structure of the story, with its inversion of progress and regress, lends additional irony to the resolution, when the old people resolve to embark on what the reader knows will be a fruitless quest for the Fountain of Youth.

The dramatic climax of a short story occurs when the central conflict is resolved with one opposing force winning over the other. When Dr. Heidegger gives water from the Fountain of Youth to his guests to restore their youth, he offers them some advice:

Before you drink, my respectable old friends . . . it would be well that, with the experience of a lifetime to direct you, you should draw up a few general rules for your guidance, in passing a second time through the perils of youth. Think what a sin and shame it would be if, with your peculiar advantages, you should not become patterns of virtue and wisdom to all the young people of the age!

His friends respond with "a feeble and tremulous laugh" to think that with all their years of living they would "ever go astray again." They are confident they have learned from all their mistakes and, having a second chance to be young, they will act far more wisely. The conflict is established. Will they behave as "patterns of virtue and wisdom" when they become young again, or will they revert to their former selves? Will they really have learned from experience, or will their basic natures determine their actions?

The conflict is resolved when Heidegger's friends regain their youth and immediately revert to their prior youthful behavior. The Widow Wycherly becomes flirtatious; Killigrew, Gascoigne, and Medbourne, filled again with "burning passions" to the point of "madness," fight each other physically for her attention. It is at this point that the story reaches its climax:

. . . they grappled fiercely at one another's throats. As they struggled to and fro, the table was overturned, and the vase dashed into a thousand fragments. The precious Water of Youth flowed in a bright stream across the floor . . . .

The conflict is resolved; clearly the four old people have not learned from experience or benefited from their mistakes in life. As a result, the Water of Youth is lost. When they quickly turn again into their elderly selves, no precious water remains to free them from old age. They are drawn "far down into the chill and darksome vale of years."