Recent technology developments in bio-logging enable researchers to monitor physiological parameters such as body temperature (Tb) and heart rate (HR) in undisturbed settings. In Norway domestic sheep graze free in the forest and on mountain pastures during the summer. This semi-domestic setting provides an excellent opportunity to validate quality, and use of bio-loggers. Thirty HR loggers (Star Oddi, Iceland) were implanted subcutaneously in twenty lambs in two different flocks, and in 10 ewes in one of the flocks. Of the 30 loggers, 22 could be retrieved and the data downloaded. The HR was measured based on a 3 sec ECG strips with 1, 2 or 10 minute intervals. Lambs and ewes were observed on the summer pasture and behavior was recorded. Mean active HR for lambs was 137 bpm (sd 16 bpm), and 103 bpm for ewes (sd 15 bpm). Mean passive HR of lambs was 110 bpm (sd 14 bpm) and 100 bpm for ewes (sd 13 bpm). During three periods, distributed over the summer, the loggers stored the raw ECG data enabling validation of the heart rate data. The sensing quality of 1700 ECG strips was manually assigned and RR intervals of 600 randomly selected manually measured. The HR logger assessed the quality in >99% of the measurements correctly and the difference between the manual measurements and the loggers HR was not different to 0 (t=1.3, p=0.2). We conclude that these HR loggers are suitable for monitoring HR in free ranging large mammals.