A pyrido [3,4-d] pyridazine derivative is provided herein of the formula: ##SPC1## Wherein R.sub.1 is hydrogen or an alkyl group of 1 to 4 carbon atoms; R.sub.2 is an alkyl group of 1 to 4 carbon atoms, an aralkyl group of 7 to 8 carbon atoms, or a substituted or unsubstituted phenyl, naphthyl, furyl or pyridyl, the substituent being a lower alkoxy group of 1 to 3 carbon atoms, a lower alkyl group of 1 to 3 carbon atoms, halogen or nitro; R.sub.3 is hydrogen or a lower alkoxy group of 1 to 4 carbon atoms; and R.sub.4 is a substituted or unsubstituted morpholino, piperidino or pyrrolidino, the substituent being methyl or ethyl and a pharmaceutically acceptable salt thereof. The compounds of the present invention have effective and strong diuretic action and exhibit extremely low toxicity characteristics.