Abstract: A non-toxic drilling fluid (drilling mud), for use in rotary-type well-drilling operations, comprises a non-toxic base-oil consisting essentially of branched-chain paraffins which may or may not contain ester functionalities, the base-oil having: (i) between about 16 and about 40 atoms per molecule, (ii) a kinematic viscosity, at 40.degree. C., of between about 1 and about 16.5 centistokes; (iii) a flash point of at least about 90.degree. C.; and (iv) a pour point of less than about -40.degree. C. Preferably, the branched-chain paraffin used as the base-oil consists essentially of the dimer of 1-decene, which has a viscosity of about 5 centistokes at 40.degree. C. and which has 20 carbon atoms per molecule. The drilling fluid can be about 100 percent base-oil or can be an emulsion of the base-oil in water, in which case the amount of base-oil is at least about 30 volume percent, and is more preferably between about 65 and about 95 volume percent.