The effect of different lipids on the fertilizing capacity of Bufo arenarum spermatozoa and on acrosome breakdown of Leptodactylus chaquensis spermatozoa was studied. Sonicated vesicles of egg yolk phosphatidylcholine (1 mM) were as effective as vesicles of egg yolk phosphatidylcholine:cholesterol (molar ratio 1:0.9) in inhibiting the fertilizing capacity of Bufo arenarum spermatozoa. This suggests that cholesterol depletion from the spermatozoa was not the cause of the fertility loss. Bufo arenarum spermatozoa were incubated with phosphatidylcholines with even chain length from 6 to 18 carbons. At a concentration of 0.01 mM, didecanoyl-phosphatidylcholine reduced fertilizing capacity to 10% in a few minutes and to 0% within 60 minutes. Didodecanoyl-phosphatidylcholine required 2 hours to reduce fertility to 10% and 4 hours to cause a 100% loss of fertilizing capacity. A concentration of didecanoyl-phosphatidylcholine as low as 5 x 10(-4) mM caused a more than 95% fertility loss in less than five minutes. At a concentration of 0.1 mM, didecanoyl-phosphatidylcholine induced complete acrosome breakdown in Leptodactylus chaquensis spermatozoa in 15 minutes, whereas didodecyl-phospatidylcholine required 2 hours. At a concentration 100-fold lower didecanoyl-phosphatidylcholine induced complete acrosome breakdown in 2 hours. Electron microscopic observations in both species showed loss of acrosome caused by the action of the didecanoyl-phosphatidylcholine. Longer chain phosphatidylcholines exerted an inhibitory effect on Bufo arenarum spermatozoa fertilizing capacity at a higher concentration when in a vesicular form.