Concentrated CaCl and CaBr salt solutions of densities up to 2.3 kg L are regularly used to control hydrostatic pressure in oil wells during special operations in the exploration and production of natural gas and crude oil. Various concentrations of high density salts are frequently left in mud pits near the drilling site as waste, polluting fresh and ground waters by spillage and drainage. The toxic effects of these salts have already been observed. This study investigated the effects of CaCl and CaBr on water flea Daphnia magna Straus in a 21-day reproduction test. The three tested concentrations of CaCl (240, 481, and 1925 mg L ) caused a significant dose-response decrease of reproduction (p<0.001). With CaBr (533 and 1066 mg L ), only aborted eggs were produced, demonstrating the embryotoxicity of the substance. The results suggest that high concentrations of the tested chemicals are harmful to Daphnia’s reproduction and could reduce its abundance. Koncentrirane otopine soli CaCl i CaBr gustoće do 2,3 kg L redovito se koriste za kontrolu hidrostatskoga tlaka u bušotinama tijekom posebnih operacija u istraživanju i proizvodnji prirodnoga plina i sirove nafte. Različite koncentracije soli visoke gustoće često su bile ostavljene u isplačnim jamama u blizini područja bušenja te su prelijevajući se ili ocjeđujući u podzemlje zagađivale slatke i podzemne vode. Iako je dosad već provedeno nekoliko istraživanja toksičnoga učinka koncentriranih otopina soli CaCl i CaBr , u ovom radu istraživan je učinak CaCl i CaBr na vodenbuhu Daphnia magna u dvadesetjednodnevnom testu reprodukcije. Tri ispitane koncentracije CaCl (1925, 481 i 240 mg L ) prouzročile su značajan, o dozi ovisan pad reprodukcije (p<0,001). U otopinama CaBr (533 i 1066 mg L ) proizvedena su samo abortirana jaja, što pokazuje embriotoksičnost te tvari. Rezultati upućuju na to da su visoke koncentracije ispitivanih tvari štetne za razmnožavanje vodenbuhe te mogu smanjiti brojnost tih organizama.