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In the human body aldosterone plays a major role in the regulation of the salt water balance and the blood pressure. To investigate the pathophysiological effects of aldosterone in a mouse model appropriate drug delivery systems, that release the drug in a constant and continuous manner, are needed. Therefore novel in situ forming implants were developed as alternative aldosterone releasing depots. The non-invasive analytical method electron spin resonance (ESR) spectroscopy was applied to characterize in situ the implant formation by direct and continuous quantification of polymer precipitation and solvent exchange rates. Therewith influences of several key formulation parameters, such as type of the solvent and the polymer have been investigated. In this context AB di- and triblock copolymers of poly(ethylene glycol) (PEG) and poly(lactide-co-glycolide) (PLGA) were firstly explored as polymeric matrices for in situ forming implants. The phase separation kinetics and therewith the aldosterone releases were highly dependent on the hydrophilic character and the molecular weight of the used polymers. [ABSTRACT FROM AUTHOR]