The aim of present studies was to examine the interrelationships between reproductive events, age, body mass and steroid hormones in roe deer females (Capreolus capreolus). For this purpose we compared seasonal changes in body mass, blood levels of progesterone and estradiol (1) in young (1 year) and adult (2-4 years old) does and (2) in pregnant and non-pregnant animals. Monthly during 12 months all animals were weighed, blood plasma was collected, and concentration of progesterone and estradiol was analysed by RIA. Pregnant animals had significantly higher body weight, than non-pregnant ones, in November (before foetus implantation), and lower body weight in comparison with non-pregnant females in August (after parturition). In non-pregnant females high level of progesterone was observed from August (mating) up to December. Thereafter progesterone level declined up to minimum in summer months (April-July). Pregnant animals had increased progesterone level from February (foetus implantation) up to June (time after labour). In non-pregnant females, three peaks of estradiol concentration were observed in October, December and May. Pregnant animals, in contrast to non-pregnant females, had spring (January-March) gravidity-associated peak of estradiol level, but absence of summer (May) peak before parturition. Comparison of annual changes in body weight and plasma steroid hormone level in pregnant yearlings and old animals, as well as the number of offspring in these animals did not show principal age-dependent differences in these indexes, although yearlings had higher absolute progesterone (in December) and estradiol (in October and November) level than old animals. Our observations suggest significant seasonal changes in plasma progesterone and estradiol level and body weight in this species. Substantial differences in these changes in pregnant and non-pregnant animals demonstrate the involvement of steroid hormones in control of pregnancy in roe deer does. The absence of age-dependent differences in body weight and fecundity rate do not confirm previous hypothesis that age-dependent differences in metabolism and body mass can reduce fertility rate in yearlings. Moreover, our observations are the first demonstration of higher rate of steroidogenesis in young animals, than in adult females during early stages of gravidity and before embryo implantation. It is not to be excluded, that age-dependent reduction in ovarian steroid hormones level could be a cause of future infertility in old animals.