Many synthetic glucocorticoids from medical applications occur in the aquatic environment. Whether they pose a risk for fish health is poorly known. Here we investigate effects of glucocorticoids fluticasone propionate (FLU) and triamcinolone acetonide (TRI) as single steroids and as ternary mixtures with clobetasol propionate (CLO) in zebrafish embryos. Exposure to FLU and TRI in a range of concentrations between 0.099 and 120.08 mu g/L led to concentration-related decrease in muscle contractions and increase in heart rate at 0.98 and 1.05 mu g/L, respectively, and higher. Genes encoding for proteins related to glucose metabolism (g6pca, pepck1), immune system regulation (fkbp5, irg1l, socs3, gilz) and matrix metalloproteinases mmp-9 and mmp-13 showed expressional alterations, as well as genes encoding for the progestin receptor (pgr) and corticosteroid dehydrogenase (hsd11b2). FLU accelerated hatching and led to embryotoxicity (immobilization and edema). Ternary mixtures (FLU + TRI + CLO) induced the same physiological and toxicological effects at concentrations of individual glucocorticoids of 11.1-16.37 mu g/L and higher. Heart rate was increased in the mixture at concentrations as low as 0.0885 - 0.11 mu g/L of each steroid. Glucocorticoids in mixtures showed additive activity; the fold-changes of transcripts of 19 target genes were additive. Together, our data show that glucocorticoids act additively and their joint activity may be of concern for developing fish in contaminated environments.