A high percentage of the heads of mouse, hamster, and rabbit spermatozoa were detached from their tails by vortexing a suspension of spermatozoa that had been incubated with dithiothreitol (DTT), a sulfhydryl reagent. Similar treatment of spermatozoa that had been aged under physiologic conditions for several hours and were still motile before reaction with DDT gave a much lower percentage of head detachment. This stabilization of the sperm head-tail junction developed only if spermatozoa were motile during aging. n-Butylamine also induced head detachment of mouse spermatozoa. Motile and immotile mouse spermatozoa became resistant to n-butylamine when aged in physiologic medium and buffer, respectively. However, stabilization developed much faster if spermatozoa were motile. Stabilization of the head-tail junction of motile spermatozoa to the reagents occurred over the time periods required for capacitation and the development of hyperactivated motility. Development of resistance to the two reagents may be biochemical events associated with capacitation.