The extensive use of pharmaceuticals and their poor removal by wastewater treatment plants has led to the emergence of pharmaceutical compounds as global aquatic contaminants. Progestins, the synthetic analogues to progesterone (P4), are receiving increasing attention as contaminants and have been shown to impair reproduction in fish and amphibians at low ng L-1 concentrations. Certain progestins have androgenic properties and are several orders of magnitude more potent in terms of reproductive impairment in fish than non-androgenic progestins. To characterize the androgenic effects of progestins in fish, adult three-spined sticklebacks (Gasterosteus aculeatus) and zebrafish (Danio rerio) larvae were exposed to progestins via the ambient water. In female sticklebacks, the androgenic progestins levonorgestrel (LNG) and norethindrone (NET) induced production of the androgenic biomarker protein spiggin and reduced production of the egg yolk protein vitellogenin. Comparison with well-known environmental androgens showed that LNG and NET, with regard to spiggin induction and vitellogenin induction, are among the most potent environmental androgens known. In male sticklebacks, LNG inhibited the post-breeding regression of secondary sex characters and spiggin production, as well as the resumption of spermatogenesis, functionally inhibiting the natural transition from breeding into non-breeding condition. Exposure of zebrafish larvae to LNG caused all fish to develop into males, whose sexual development was also significantly accelerated. P4 had no effect on the sex ratio, while slightly accelerating sexual development at high concentrations. Suppression of vitellogenesis in females, disruption of the male reproductive cycle, male-biased sex ratios and precious male puberty could all entail severe fitness costs and severely affect fish populations. Most of the effects of androgenic progestins in this thesis occurred at levels within the range of reported environmental levels, and may therefore occur in progestin-contaminated waters. In conclusion, the present results establish LNG and NET as highly potent androgenic pollutants of environmental concern, and provide strong support to the contention that the reproductive impairment in fish caused by progestins is chiefly mediated by their androgenic properties.