Abstract
1. Sperm tails were isolated from mechanically disrupted spermatozoa of Perca fluviatilis and suspended in buffered sucrose solution. When freshly prepared suspensions were treated with ATP and magnesium chloride and centrifugal pellets obtained these weighed up to about 40% heavier than controls. 2. The effect is closely linked to the enzymic splitting of ATP and under conditions where this was inhibited no increase in weight was observed. Pellets from suspensions aged overnight at 2 degrees and then treated with ATP and magnesium chloride were no heavier than the controls although the suspensions had undiminished enzymic activity. Sperm tails from Salmo trutta did not show the effect. 3. Analyses showed that the weight increases were the result of uptake of sucrose solution and that sucrose completely penetrated the pellet water. Polyethylene glycol (mol.wt. 4000) did not penetrate the water of the pellets completely and an amount approximately equal to 1.9 times the flagellar dry weight was inaccessible. 4. Electron-microscopic studies indicated that the effect is the result of tail disintegration.