To enhance our understanding of how aquatic endotherms respond thermally to their environment, we present two new designs for recording heat flux (HF) and skin temperature (T s) from the dorsal fins of free-ranging dolphins. The first package, held on by a series of small suction cups, was designed for short-term deployments on the order of hours. The second package, a long-term attachment, was held on via two 5.0-mm -Delrin® pins and was designed for deployments on the order of days. The former version was configured to release remotely, by way of a galvanic linkage, removing the need to re-capture tagged dolphins. The latter was designed to be removed after re-capture of the individual but also included a fail–safe galvanic release. One of the major advances of our approach is that it allows both convective and conductive pathways of heat loss to be dynamically recorded from free-swimming wild dolphins, whereas previous approaches have recorded primarily conductive heat loss from trained captive or restrained wild dolphins. The short-term, re-usable package has been field-tested on 55 bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida and the long-term device on two spotted dolphins (Stenella attenuata) in the eastern tropical Pacific Ocean. By also incorporating into each package a time–depth–velocity recorder we were able to examine patterns of heat flux in relation to changing behavioural states of the tagged individual. Heat flux recorded at the dorsal fin was found to be individually variable and dynamic. Water temperature (T w) had a profound effect on patterns of heat loss in both species with HF increasing with even minor decreases in T w. This device significantly improves our ability to understand the thermal biology of dolphins as they move about in their natural environment.