Water is the principal limiting resource for plants in Californian mediterranean-climate plants. The effect of rainfall variation on the serpentine grassland plant community was investigated at the Jasper Ridge Biological Preserve near Stanford University. The effect of rainfall variation during germination and establishment of annual species was compared for the 1983-84 and 1986-87 growing seasons. By following marked seedlings from germination to reproduction, mortality was related to rainfall pattern on a weekly time scale. It was found that peaks of mortality followed rain-free weeks. The timing and rate of mortality differed among species. Although rainfall varied between years, the pattern remained. An early rainfall event in 1986-87 initiated germination, but lack of follow-up rains resulted in very low survival. The effect of the predictable summer rain-free period was examined by growing four species of plants, representative of the phenological and life form diversity of the community, in the greenhouse. Sequential harvests revealed a remarkable similarity in both grass and forb species during the rainy part of the growing season. As the plants entered the rain-free period, significant differences were found between annual and perennial species. The various life form-phenological groups appear to represent a continuum, distinguished by the fate of below-ground carbon stores accumulated during the rainy part of the season. Five years of data (including four consecutive drought years) from a census of plant species on a 100 m transect were used to explore the effects of inter-annual variation in rainfall on species composition and spatial dispersion of plants on adjacent serpentine and greenstone grassland. For non-native annual grass species, there was a significant interaction between substrate and drought on both species abundances and spatial dispersion of plants on serpentine soil. A conceptual model is proposed to explicitly relate rainfall variation to stages in the establishment of the serpentine grassland plant community. Environmental constraints on plant growth potential are used to divide the year into four time periods and to characterize the annual rainfall pattern in terms of plant growth processes.