The relative ability of the underground buds of a Mediterranean perennial grass species to break dormancy in response to summer rain was compared between individual plants and between half-sib families in two experiments with broadly-based breeding populations under different environmental conditions. Significant genetic variation was found in summer regrowth activity in both experiments. The narrow-sense heritability of the ratio of new to old, dead tillers after summer rain under spaced plant conditions at a relatively cool site was 0.40±0.16. Under sward conditions at a warm site, the narrow-sense heritability of an index of summer regrowth potential on a half-sib family mean basis was 0.36±0.08. Summer regrowth activity was negatively but weakly correlated with flowering time (rg=−0.22), and strongly positively correlated with early autumn herbage yield (rg=0.79). Summer regrowth activity was not genetically correlated with seedling size or herbage yield in the second or third growing seasons. These correlations are different in sign or magnitude from those observed in collections of Mediterranean ecotypes from which the breeding populations were derived by crossing to Australian cultivars and recurrent selection for vigour and persistence. The correlations in Mediterranean populations therefore are not due to pleiotropic effects; they can be altered by recombination. Phenotypic variance relative to the population mean was considerable in both experiments, and hence the level of the responsiveness of dormant buds to summer rainfall should be manipulable readily by selection. The possible effects of altered responsiveness on long term survival and productivity in several different environments are discussed.