A quantitative description of the copulatory behaviour of two species of closely related voles (M. agrestis and M. arvalis) is presented. Special attention is paid to the separate contributions of males and females to the time course of events in order to make predictions about the problems which would arise in interspecific copulation attempts. The species were similar with regard to mounting latency, number of ejaculations and durations of pauses between mounting series after ejaculation. Durations of mounting series were shorter in M. agrestis than in M. arvalis. The duration of a series is determined by the number and length of intermount intervals (IMIs) and nonejaculatory mounts and duration of the ejaculatory mount. The number of mounts was greatest and their duration was shortest in M. arvalis, leading to equal total mounting time in the two species. IMIs were longest in M. arvalis and, because the number was also greatest in M. arvalis, the total IMI time per series was longest in M. arvalis. Consequently, the mounting series was longest in M. arvalis. Durations of mounts were exponentially distributed; therefore, the tendency of males and females to terminate mounts could be estimated as constants. In both species, females showed a much higher tendency to terminate mounts than did males. In M. arvalis, both males and females showed higher termination tendencies than in M. agrestis. Males of the two species did not differ in their tendencies to terminate IMIs. Females of M. arvalis, however, aggressively resisted mounting more often than did M. agrestis females, leading to relatively long IMIs in M. arvalis. M. agrestis males often ejaculated during a single mount; M. arvalis males did so after at least five mounts. In accord with this species difference, it could be shown that mount terminations (or the experiences connected with them) negatively influenced the excitation state of M. agrestis males; i.e., the number of mounts prior to an ejaculation was greater (and the total mounting time was longer), the shorter the time the mounts lasted (i.e., the higher the females termination tendency was). In M. arvalis the number of mounts was smaller, the shorter the mounts lasted. In this species, high termination tendencies of the female correlated with much resistance to mounting (long IMIs). The combination of short mounts with long IMIs seems to have a positive influence upon the males. Thus, M. agrestis females delay ejaculation by a high termination tendency of mounts, whereas M. arvalis females seem to accelerate ejaculation by similar actions. Males and females appear to be very well adapted to each other: the M. agrestis female is relatively tolerant and the male ejaculates sooner, the more tolerant she is; the M. arvalis female is rejective and the male ejaculates sooner the more rejective she is, at least within the limits of rejectivity of the observed females. Females differ such that rejectivity in M. agrestis equals tolerance in M. arvalis, so that interspecific copulations are not likely to be successful. It is hypothesized that the rejectivity in M. arvalis fits in with the gregarious way of life of this species. For the female may select the fittest males and/or rejectivity by the female and the positive effect of it upon the male may serve as an incest preventing mechanism.