Adult spatial and social relationships in a marked population of black-lipped pikas (Ochotona curzoniae) were studied during the reproductive season from April to July near Qinghai Lake, People's Republic of China. Families of pikas occupied relatively discrete home ranges that contained a burrow system with many entrances in continuous alpine-meadow habitat. Some burrow systems contained a monogamous pair of breeding adults; in others one male controlled access to two or more females. During the breeding period, adult males vigorously guarded females with which they were associated. Young from three sequential large litters occupied their natal home ranges and did not disperse during the reproductive season. Density on individual home ranges increased throughout the study, and population density reached ca. 300 animals/ha following emergence of the third litter. Most interactions among adults involved reproduction (following, approaching, mating) and occurred early in the summer. Most social interactions were affiliative and were between adults and juveniles within family home ranges. Most of these interactions involved adult males. Adult males also were responsible for most other behaviors that could be associated with vigilance and awareness of the social milieu on the meadow and with maintenance of home-range quality. There was no indication that rate of affiliative behavior differed among males from variable mating systems.