The article presents description of the most important parenteral forms, which release an active substance in sustained manner. Technological and biopharmaceutical information is supplemented with examples of commercial products, currently registered in Poland. Mechanism of drug release from the dosage form and duration of the process, role of other excipients and characteristics of certain medicinal products is presented. The information about suspensions and oily solutions for injection is summarized. Owing to specific chemical modifications and selection of suitable excipients, it was possible to develop these types of medicinal products for some antibiotics, hormones, antipsychotics and cytostatics. These formulations, after subcutaneous or intramuscular injection, release the active substances even for several weeks allowing to reduce the frequency of drug administration and finally help to improve patient's compliance. Here also the modified time course of insulin products achieved by selection of appropriate suspension form or insulin analogue is discussed.