The action of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) on the isolated heart of the frog (Rana ridibunda) and two insects, the honeybee (Apis mellifera macedonica) and the beetle (Tenebrio molitor), was investigated using basic electrophysiological methods. The results of this study showed that a concentration of 1 μM 2,4-D was required to reduce the force and the frequency of the isolated heart of the honeybee to about 70% of the initial contraction in less than 20 min. To cause the same effects on the atria of the frog, 45 μM 2,4-D was required and on the isolated heart of the beetle, over 1000 μM of 2,4-D. The presence of an extensive system of gap junctions found in the honeybee is most probably the cause of the unusual sensitivity of its heart to 2,4-D, compared with the heart of the beetle, where no gap junctions were identified.