Tröger's-base analogues bearing fused pyrazolic or pyrimidinic rings were prepared in acceptable to good yields through the reaction of 3-alkyl-5-amino-1-arylpyrazoles and 6-aminopyrimidin-4(3H)-ones with formaldehyde under mild conditions (i.e., in ethanol at 50 °C in the presence of catalytic amounts of acetic acid). Two key intermediates were isolated from the reaction mixtures, which helped us to suggest a sequence of steps for the formation of the Tröger's bases obtained. The structures of the products were assigned by 1H and 13C NMR, mass spectra and elemental analysis and confirmed by X-ray diffraction for one of the obtained compounds.