5-Pyridin-4-yl-1,3,4-oxadiazole-2-thiol (2) was obtained from the reaction of isonicotinic acid hydrazide with carbon disulfide in basic media and converted into 4-amino-5-pyridin-4-yl-4H-1,2,4-triazole-3-thiol (5) by the treatment with hydrazine hydrate. The synthesis of 3 and 6 was performed from the reaction of 2 and 5 with ethyl bromide. The treatment of 5 with 4-fluorobenzaldehyde or indol-3-carbaldehyde resulted in the formation of 4-[(arylmethylene)amino]-5-pyridin-4-yl-4H-1,2,4-triazole-3-thiols (7a and 7b). The reactions of 2, 5 and 7a with some primary and secondary amines in the presence of formaldehyde afforded the corresponding Mannich bases, 4a, 4b, 9a-9c and 8. All newly synthesized compounds were screened for their antimicrobial activity. The antimicrobial activity study revealed that all the compounds screened showed good or moderate activity except compounds 2, 7a, 7b, 8 and 9b.