Studies of reproductive organs, fat body and liver in relation to different activity periods were made to describe the breeding biology of the female adder, Vipera berus, in SW Sweden. The female adder is a biennial breeder. Vitellogenesis and formation of preovulatory follicles start in autumn the non-reproductive year and continue to the end of May the following spring when ovulation takes place. Copulation precedes ovulation and sperm is found in uterus a month before ovulation. Primary oocytes were seen after ovulation in the reproductive year. Atresia were seen more frequently during reproductive years and in all classes of follicles. Clutch size (X=8.8) had a linear increase with female body size and juveniles were born in early August. Thecal gland cells increased in number and the epithelium of the uterus became hypertrophied late in the non-reproductive year and early in the reproductive year. The weight of the ovary, fat body and liver went through a cyclic pattern with the highest peak in the reproductive spring. Fat body and liver weights were lowest at the end of the gestation period. Feeding periods, fat mobilization, reproductive effort, endocrine ectivity and adaptive value of the biennial reproduction are discussed. The adaptation to short season habitats is stressed.