Modern thin-layer chromatography (TLC) was used for the evaluation of delta2-oxazolines-1,3 I and N-acylaziridine VII structures, as potential proinsecticides of carboxylic acids III. Thus the unmasking2 of the active principles III from delta2-oxazolines-1,3 Ia-c and N-acylaziridine VIIc was monitored by spotting aliquots directly onto RP-18 TLC plates, without any sample pretreatment during in vitro assays performed in concentrated locust tissues. To achieve a good separation of carboxylate IIIa from endogenous components of the tissues, a short preliminary development with methanol or ion-pairing was necessary. From UV-TLC chromatograms (densitograms) it appeared that in a phosphate buffer at pH 7.4, the oxazoline Ia with a C2 substituent devoid of alpha-ramification or alpha,beta-insaturation hydrolysed slowly into the corresponding beta-hydroxylamide VIa and intermediate aminoester Va. Significantly, locust mesenteron (or fat body) efficiently triggered the unmasking of IIIa, a transformation which corresponds to the expected proinsecticide behavior of Ia. Conducting TLC monitoring in the same locust tissues also revealed that the oxazolines Ib and Ic with an alpha-ramification and an alpha,beta-insaturation, respectively, cannot be considered as proinsecticides of the corresponding carboxylic acids IIIb and IIIc. In contrast, the N-acylaziridine VIIc appeared as a convenient proinsecticide structure for masking the carboxylic acid IIIc.