Much effort is being made to establish relationships between the molecular events that take place in spermatozoa under fertilizing conditions and actual sperm function during fertilization. During capacitation, the process that 'primes' spermatozoa for interaction with the egg, components of the sperm's environment, notably bicarbonate, provoke various specific changes in the architecture and functioning of the sperm plasma membrane in a large number of cells. The individual changes have been found to proceed on different time scales, and may therefore represent sequential stages in the capacitation process. However, each change takes place at different rates in individual cells, revealing considerable functional heterogeneity within the sperm population. Recent work on membrane changes provoked by cooling has indicated similarities with capacitational changes. The effect of cooling may therefore be to induce premature capacitation (and destabilization). Such an effect would greatly compromise sperm fertilizing potential. A pig sperm-egg interaction model was used to examine quantitative details of zona binding and zona penetrating abilities within capacitated sperm populations, and sperm behaviour was found not to accord with generally held beliefs. In particular, individual spermatozoa that have bound to the zona pellucida show great variation in the delay before penetrating: no evidence has been found for a specially competent subgroup. Even in sperm samples incubated to undergo maximal capacitational membrane changes, cells with actual penetrating potential represent less than 15% of the total number that attach initially to the zona pellucida. Thus detection of capacitational membrane changes appears greatly to overestimate zona penetrating capability. Future studies linking sperm membrane characteristics with semen fertility in the field will need to consider differences between in vitro and in vivo conditions. The need for survival in the female tract may require much slower sperm responses than are considered optimal for in vitro fertilization.