Introduction: Prostate cancer is the most common type of cancer in Sweden and is often treated using antiandrogenic drug therapy. Two substances belonging to this class of pharmaceuticals are bicalutamide and flutamide. After excretion from the human body, the drug molecules enter the wastewater treatment plant (WWTP). The WWTPs are not effective enough to completely remove pharmaceutical residues, why presence of both bicalutamide and flutamide can be detected in WWTP effluent water. Previous findings: Antiandrogens have been reported to affect reproduction in adult fish, but studies regarding possible effects on the embryonic development of fish are few. Aim: The present study sought to investigate if exposure to bicalutamide or flutamide cause toxicity in the early developmental stages of zebrafish embryos, and whether negative effects occur within concentrations relevant to measured environmental levels. Method: A modified OECD FET-test was used, where additional sublethal endpoints were included and the time period for assessment extended to 144 hours post fertilization (hpf). In addition, a locomotor activity assay was performed at 144 hpf in order to observe any sub-lethal swimming behavioral effects. Results: High doses (10 mg/L) of flutamide led to 100% lethality of the zebrafish embryos but the results suggest no acute toxic effects in the high dose treatment group of bicalutamide, or of either flutamide or bicalutamide within in the low (0.1 mg/L) or intermediate (1 mg/L) treatment groups. Neither did the locomotor activity assay result in statistically significant results, although the pattern of swimming activity in the low dose groups suggests that behavioral developmental effects could be present. Conclusions: High doses of flutamide caused mortality of the embryos, but no lethal or sublethal effects were present at environmentally relevant concentrations. The modest outcome of present study however suggests that further investigation of behavioral developmental effects of antiandrogens could be of future relevance. Analysis of the expression of genes related to neuronal growth, memory and other cognitive behaviors associated with behavioral changes, would then be of interest for further studies.