The effect of noradrenaline and isoprenaline on cyclic AMP accumulation has been investigated in primary rat astrocytes which contain either (a) protoplasmic astrocytes alone or (b) both fibrous and protoplasmic astrocytes. Isoprenaline and noradrenaline stimulated cyclic AMP formation in both astrocyte culture preparations. Combinations of noradrenaline (1 microM) and isoprenaline (1 microM) produced a cyclic AMP response which was 58% and 26% of that produced by isoprenaline alone in protoplasmic and mixed fibrous/protoplasmic cultures, respectively. In both preparations this inhibitory effect of noradrenaline was antagonized by the alpha 2-adrenoceptor antagonist yohimbine (1 microM). A striking feature of the concentration-response curve for isoprenaline (EC50 = 0.8 microM) in mixed fibrous/protoplasmic cultures was that the cyclic AMP response decreased sharply at concentrations above 1 microM. This phenomenon was not seen in cultures containing protoplasmic astroglia alone. The fall in the isoprenaline concentration-response curve was not observed in the presence of the alpha-adrenoceptor antagonist phentolamine (1 microM), the dihydropyridine calcium antagonist isradipine (10 microM), the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.1 mM) or in nominally calcium-free medium. The effect of phentolamine was mimicked by the alpha 1-adrenoceptor antagonist prazosin (1 microM) but not by the alpha 2-antagonist yohimbine (1 microM). In conclusion, the data from this study suggest that two different populations of astrocytes in in vitro culture are able to raise intracellular cyclic AMP levels via beta-adrenoceptor activation and that there are differences in the extent of alpha-adrenoceptor (both alpha 1- and alpha 2-) mediated inhibition of cyclic AMP accumulation between the two primary astroglial cell preparations.