New methods are needed for rapid and sensitive assessment of sperm function. As the ability to fertilize an oocyte is acquired during the capacitation process, assessments of sperm function have to be performed under fertilizing conditions. In this study, we monitored the dynamics of the temporal response of sperm from ejaculates of both fertile and subfertile boars to capacitating conditions in vitro (responsiveness) by following the changes in the response to calcium ionophore treatment and in [Ca(2+)](i). The differences between individual males were also investigated. Ionophore-induced changes and increased intracellular calcium ion content in boar spermatozoa were found to progress as a function of time during incubation under capacitating conditions. After primary kinetic analysis, 120 min was chosen as the point in time for assessment of responsiveness. Intra-boar variability in responsiveness parameters was relatively high (variation coefficient CV>30%), especially in the response to ionophore treatment, indicating that an isolated test may be inadequate for the evaluation of sperm function. Despite this high variability, there were markedly significant individual differences with respect to changes during capacitation, and there were significant correlations between conventional and responsiveness sperm parameters. The population of samples from subfertile boars, was found to be heterogeneous in regard to sperm responsiveness to capacitating conditions. There were two significantly different classes of subfertile boars ("low" and "high" responders), indicating that fertility may be associated with suboptimal rather than maximal response (both too rapid and too slow membrane changes). Therefore, criteria for quality judgement should include both the low and upper limits of responsiveness. The use of responsiveness parameters together with conventional spermatological parameters improved the prediction level of multiple regression models for farrowing rate and litter size. It can be concluded that the combination of sperm responsiveness parameters applied here is a suitable tool for the evaluation of sperm function.