The synthesis of 1-acyl-3-arylimidazolidines were performed by reaction of N-acyl-N -arylethylenediamines with formaldehyde and Montmorillonite clay K-10 as a catalyst under microwaveirradiation. INTRODUCTION Tetrahydroimidazoles (imidazolidines) are cyclic aminals of pharmacological interest due to the bioactivity shown by some members which is closely related to the substitution type. The most studied