We analyzed data collected from 19 428 necropsied brown lemmings (Lemmus trimucronatus) collected over a period of 22 years near Barrow, Alaska. Here we report on seasonal and cyclic patterns in reproductive activity of females and males, population structure, adult body size, condition (weight per unit length and body fat), and wounding. We also provide the first detailed information on these characteristics at low population densities. The seasonal pattern of reproduction included short summers (about 2.5 months) with intense reproduction, long winters (about 6.5 months) with moderate reproduction, and virtual cessation of breeding during the short springs and autumns (1.5 months each). Overall sex ratios appeared equal, but the proportion of males decreased during summer. The proportion of immature lemmings increased in late summer and autumn after intense summer breeding, then declined in winter. Adult body weight and condition (weight per unit length) peaked in summer when the food supply also peaked, but body fat, though still minimal, peaked in winter. The greatest incidence of wounding occurred in males, but wounding increased in both sexes during early summer, the period of most intense breeding. As reported previously for arvicoline rodents, a longer summer reproductive season occurred during the low and increase phases, but no change in litter size occurred during the cycle. Contrary to earlier reports, a much lower frequency of pregnancy occurred during the high phase, but later reproductive maturation during the high phase received little support. The sex ratio generally changed little during the course of a population cycle, but the proportion of females did decline during springs when the population increased to high densities. We found a greater proportion of adults during the high and decreasing phases, reflecting the reduced reproduction during those phases. Higher body weights and better condition of adults occurred during high densities. Wounding (fresh scars from bites) also peaked during years with high densities, though only for adult males and non-reproductive females. Overall, it appeared that nutrition strongly influenced seasonal changes in reproduction, population structure, and body condition; predation affected both seasonal and cyclic changes in population structure and survival; and that nutrition, and perhaps social strife, caused cyclic changes in reproduction, survival, and body size.