SUMMARYRecent studies of the control of stomatal aperture by abscisic acid (ABA) have suggested that its cellular action depends on the availability of calcium ions, which appear to function as secondary messengers. Stimulus‐response coupling may, therefore, be similar to that found in many animal tissues.We report here preliminary evidence that ABA has a profound effect on some mammalian tissues and on a cyanobacterium. Studies on smooth muscles from the vas deferens and bladder of rat showed that 10−6 M ABA enhanced field stimulation responses by about 25 %. This effect was inhibited by the calcium‐channel blocker, nifedipine. In K+‐depolarized bladder smooth muscle in which the fast calcium channels were voltage‐inactivated, 10−6 M ABA augmented contracture tension, and enhanced the slow tonic phase of the response which is known to be dependent on the activity of slow calcium channels. Comparative studies using ABA and the synthetic calcium agonist BAY K 8644 suggested that there are close parallels in the mode of action of the two compounds. Heal smooth muscle was remarkably sensitive to ABA, showing enhancement of K+ contracture tension by up to 400% at only 10−9 to 10−8 M ABA.In cultures of Nostoc 6720, low concentrations of ABA (about 10 −7 M) caused a substantial increase in the heterocyst/vegetative cell ratio. Addition of ABA further enhanced the increase in heterocyst content caused by increases in the concentration of Ca2+ in the medium. These observations provide further support for the proposal that ABA functions as a calcium agonist, a concept consistent with similarities between the action of ABA and the calcium ionophore A23187. In cultures of Nostoc 6720, high (> 10−5 M) concentrations of ABA had an inhibitory effect, and some evidence of a similar dose‐response relationship was found in mammalian tissues.Preliminary studies using 45Ca2+ suggested that ABA stimulates the influx of calcium into both vasa deferentia and Nostoc 6720. It is concluded that ABA may function as a calcium agonist when applied to a wide range of prokaryotic and eukaryotic cells. A recent observation that it is present in the brain of some mammals suggests that this ‘plant hormone’ may be of much wider biological significance than hitherto recognized. The similar responses of plant, animal and cyanobacterial cells to ABA might indicate the presence of cellular mechanisms which have been genetically conserved through many stages of evolution.