Based on collections from 110 localities, the temperature-induced color change associated with low-grade metamorphism in conodonts is used for the first regional assessment of the degree of heating of Ordovician rocks in Scandinavia and the British Isles. Baltoscandian Platform rocks without intrusives in southern Sweden have apparently not been heated above 90°C. The autochthonous rocks in Jämtland appear to have been heated to a locally specific temperature between 110 and 400°C (perhaps even higher in some areas). The samples from the Oslo region, as well as those from the allochthon in the Trondheim region and the eastern nappes in Jämtland, suggest heating temperatures in excess of 300°C. Most collections studied from the British Isles indicate heating to more than 300°C, but those from the Girvan area and the Welsh Borderland show little, if any, thermal effects. In most cases, the conodont temperature indications fit well into, and clarify, the known geologic history of the study areas but at least two areal ‘anomalies’ are currently difficult to explain adequately.