The regulation of placental anion transport influences fetal accretion and placental homeostasis. We investigated whether efflux of 125I- or 36Cl- from multinucleated cytotrophoblast cells derived from human term placenta is regulated by one of three stimuli: (a) the calcium ionophore A23187, (b) a 'cocktail' of agents designed to raise intracellular levels of cAMP, (c) a hyposmotic solution. After loading with the appropriate isotope for 2 h and thorough washing, cells were exposed to sequential aliquots of buffer applied and removed each minute. Following an equilibration period of 5 min one of the stimuli was applied at room temperature At the end of the experiment the cells were lysed to give a lysate count which was used to express the count obtained from each aliquot as percentage efflux of that possible for that minute. The cAMP 'cocktail' and A23187 were applied for 5 min; the hyposmotic solution was applied for 10 min. The results for 125I- at 7 min showed that the mean efflux in the presence of hyposmotic shock was greater than control (5Σ7 ± 1Σ0 % min-1versus 2Σ2 ± 0Σ1 % min-1, respectively; mean ± S.E.M., n = 4 placentas). Similarly mean efflux at 6 min in the presence of A23187 was also significantly greater than control (6Σ5 ± 1Σ9 % min-1versus 2Σ6 ± 1Σ0 % min-1, respectively, n = 3 placentas). The mean efflux in the presence of the cAMP cocktail was not different from control at any time point. The results were qualitatively the same if 36Cl- was used in the place of 125I- and when the experiment was performed with 36Cl- in a HCO3- buffer gassed with CO2. Mean 125I- efflux at 6 min in response to hyposmotic challenge was 33 % less (P < 0Σ01) in the presence of 1 mM 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS) and 37 % less (P < 0Σ005) in the presence of 10 µM tamoxifen but no different if the hyposmotic solution was nominally calcium free. We conclude that there are differential effects of second messengers on anion efflux from the differentiated cytotrophoblast cells.