NOVELTY - Preparation of (-)-6-(4-aminophenyl)-4,5-dihydro-5-methylpyridazin-3-(2H)-one (I), comprises: (a) reacting racemic 6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone (II) with a chiral tartaric acid derivative; and (b) isolating (I).    USE - (I) is useful for preparing levosimendan, which is useful as cardiotonic that increases the sensitivity of the heart to calcium without causing a rise in intracellular calcium. No biological data given.    ADVANTAGE - The process provides highly-pure (I) without any recrystallization or purification, thus preventing the wastage of solvent and corresponding decrease in yield of product, hence making the process economical, high-yielding and industrially-viable. The use of di-p-anisoyl-D-tartaric acid as resolving agent causes rapid precipitation of the diastereomeric salt which in turn in a single step gives levosimendan in high yield. The solvent employed in the process is preferably a mixture with water which makes the process environmentally-friendly. The process provides levosimendan in high optical purity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) preparing levosimendan, comprising preparing (I); and converting the (-)-6-(4-aminophenyl)-5-methylpyridazin-3-(2H)-one to levosimendan;    (2) (-)-6-(4-aminophenyl)-4,5-dihydro-5-methylpyridazin-3-(2H)-one di-p-anisoyl-D-tartaric acid salt or (-)-6-(4-aminophenyl)-4,5-dihydro-5-methylpyridazin-3-(2H)-one di-p-anisoyl-L-tartaric acid salt; and    (3) process comprising (i) reacting racemic 6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone (II) with a chiral tartaric acid derivative to obtain the diastereomeric salt of (-)-6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone and the chiral tartaric acid derivative and a mother liquor enriched in (+)-6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone, (ii) converting (+)-6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone; and (iii) employing racemic 6-(4-aminophenyl)-4,5-dihydro-5-methyl-3-(2H)-pyridazinone obtained from step (ii) in above process.