The status of selenium supplementation of 123 mares and 87 foals during pregnancy, lactation and rearing was investigated at four Thoroughbred breeding farms in different regions in the southwest of Germany. Feed, blood plasma and whole blood was analysed for selenium content. The plasma glutathione peroxidase activity was determined. General health and reproductive performance data of the mares and foals were recorded by questionnaires. At one farm the selenium status of thirteen pregnant mares and foals was investigated over a period of ten months. The selenium supply of the four herds showed differences from marginal (0,09 ppm) to adequate (0,25 ppm). The mean value of plasma selenium concentration in the different fedlots related to selenium intake. However, within the herds with equal selenium intake, the plasma selenium concentration showed great individual differences. The plasma selenium concentration of unweaned foals up to four months of age was significant lower than the one of their mares. The ratio between plasma selenium concentration of mare and foal was roughly 2 : 1. No significant difference was found between the plasma selenium concentration of sound mares and foals compared to sick horses and those with reproductive disorders. The follow-up studies revealed a decrease of plasma selenium concentration in mares that were in an advanced state of pregnancy (week 40-48). The suckling foals plasma selenium concentration showed little changes. After weaning the plasma selenium concentration increased significantly. Selenium concentration in plasma and whole blood correlated closely, the ratio of the two being roughly 1,4 : 1. Both methods of analysis are equally sufficient to determine the selenium status of Thoroughbred mares and foals. However, the plasma selenium concentration does not yield reliable information on the selenium status of other tissues. As selenium is harvested partly in Erythrocytes, and changes in selenium intake will cause delayed alterations in whole blood selenium concentration (mean RBC life span 120 days), this has to be kept in mind, if whole blood is used to estimate the selenium status of horses. There was no significant relation between glutathione peroxidase activity in plasma and plasma selenium concentration. Glutathione peroxidase activity in plasma is no reliable parameter for information of selenium supply. Because of the great variability of plasma selenium concentration within the herds it is advisable to draw blood samples from different animals to ascertain the selenium status of the herd. Advisors/Committee Members: n (gender), Univ.-Prof. Dr. D. Schneider (firstReferee), Univ.-Prof. Dr. H. SOmmer (furtherReferee).