Abstract: An in-hole oil-drilling probe is mountable in a drill string to measure hydrocarbon concentrations in drilling fluid flowing around the drill string. The probe consists of a drill collar which is mountable in the drill string. A solvent, in which hydrocarbons are soluble, circulates within the drill collar through a hydrocarbon detection chamber. A membrane in the drill collar has an outer surface positioned to contact flowing drilling fluid and an inner surface positioned to contact circulating solvent within the drill collar. The membrane allows hydrocarbons to permeate from the drilling fluid into the solvent and into the hydrocarbon detection chamber with the solvent. A radiant energy source directs radiant ultra-violet energy into the hydrocarbon detection chamber. A sensor apparatus measures fluorescent energy radiating from the hydrocarbon detection chamber to determine solvent hydrocarbon concentration.