Novel inhibited biopolymer muds are reported that provide both environmental compatibility and excellent performance in drilling reactive shales. These muds employ the new dispersing agent zirconium citrate (ZrC) which provides effective rheology control for the build-up of low density solids in the mud. Laboratory experiments on Xanthan gum and Scleroglucan biopolymer field muds demonstrate the exceptional thinning power of ZrC compared to other deflocculants. Results from the application of ZrC for rheology control in Scleroglucan-potassium muds are given for two wells drilled in southern Italy. In both wells, the use of ZrC strongly reduced the need for mud dilution, compared to previously drilled wells, and proved an effective means for stabilizing rheology during stopped circulation. On account of the sharply-reduced mud volumes required, use of ZrC reduced total fluid costs by 26 and 44% for these wells.