Single intratesticular injections of glycerol have been reported to block spermatogenesis in rats without a concomitant change in the serum levels of testosterone and gonadotrophins (Wiebe & Barr 1983, 1984). The present studies were designed in order to confirm these effects and to cast light on the mechanism of action of glycerol. Groups of adult rats received intratesticular injections of either 200 microliter distilled water, glycerol or NaCl-solution. Following the injection of distilled water testicular histology appeared unaltered except for a few empty tubules around the injection site, which probably resulted from injection trauma. Injection of glycerol solution (greater than 3500 mosmol) caused partial tubule destruction. The most severely affected tubules were devoid of cells and filled with amorphous material. Serum FSH concentrations increased sharply in the first week and remained elevated during the entire study period of 8 weeks. Injection of an equi-osmolar NaCl-solution had very similar effects to glycerol on the testes and on hormone levels, suggesting that hyperosmolarity of the injected solutions rather than a specific action of the compounds caused the testicular damage. It is concluded that a single intratesticular injection of glycerol causes severe testicular damage and, in contrast to previous suggestions, appears unsuitable for male fertility regulation.