Fish have adapted extremely successfully to the extremes of the aqueous environ­ ment, with the teleosts being outstanding in this respect. Amongst the class Pisces are pelagic species which must maintain certain swimming speeds in order to remain buoyant, species which migrate over thousands of miles, abyssal species living in waters with markedly reduced oxygen content, species living in the subzero waters of the Antarctic and also the obligatory air-breathing species of the tropics. Even in a comparatively benign environment such as the relatively shallow waters over continental shelves, the lifestyle of fish species varies greatly, with sedentary benthic and pelagic shoaling species coexisting within a comparatively narrow depth-range. Clearly, widely varying physiologi­ cal demands are made on species occupying such different environments and exhibiting such different lifestyles, and the successful provision of an adequate oxygen supply to the tissues is therefore of paramount importance to the fish. It follows that the demands made on the fish heart in irrigating the gill vascula­ ture will vary greatly according to the lifestyle and habitat of a particular species, and it is therefore surprising that authors reporting physiological, pharmacologi­ cal, biochemical and morphological investigations on the hearts of a consider­ able number of cyclostome, elasmobranch and teleost species imply that their results and conclusions can be extended to "the fish heart" in general.