Spermatozoa obtained from testes, epididimydes and complete ejaculates of healthy rams during the breeding and non-breeding seasons were induced to show nuclear chromatin decondensation by controlled exposure to dithiotreitol (DTT) and sodium dodecyl sulphate (SDS) in vitro. A gradual resistance to decondensation was shown by sperm during epididymal transit, confirming a progressive increase in the prevalence of chromatinic disulphide bonds during sperm maturation in this species. A high % of stable (non-decondensed) sperm nuclei after treatment (79%) was found in semen from rams with normal fertility (64% non-return rate at first oestrus). Opposite changes were found in the semen from rams having low fertility rates (37%), as these showed only 31% of stable sperm nuclei. There were no differences in the spermiograms of these two groups. When semen from the same rams was tested during the non-breeding season, a similar relationship was found, although in both groups there was a higher % of sperm with stable nuclei than during the breeding season. The possible role of seminal plasma and of some of its constituents (e.g., zinc) on the decondensation of ram sperm nuclear chromatin was also studied. The presence of seminal plasma and the addition of zinc largely or completely inhibited the decondensation of ram sperm nuclear chromatin whilst the reverse situation was seen following the addition of chelating agents (e.g. EDTA) to the semen samples. The present results suggest that the induction of sperm nuclear decondensation by exposure to DTT and SDS under controlled conditions may provide a simple but reliable method for predicting in vitro the fertilizing ability of a ram semen sample.