Using an isotope labelling technique it has been shown that an organophosphorus insecticide methylparathion (0,0-diethyl 0-4-nitrophenyl phosphorothioate) depressed calcium uptake by sarcoplasmic reticulum isolated from rabbit hind leg muscle. The effect was significant for insecticide concentrations of 50 and 100 µM and was dose-dependent The insecticide exerted a more pronounced effect on calcium uptake in the presence of ATP in the reticulum environment than in the absence of ATP. The inhibitory action of methylparathion on Ca2+ accumulation by sarcoplasmic reticulum can cause a rise in myoplasmic free Ca2+, the essential prerequisite for contracture activation. Because methylparathion, as well as other organophosphorus insecticides, is primarily neurotoxic, evidence of non-specific effect could be important for assessing its environmental safety.