1. Quantitative autoradiographic studies were conducted to determine the distributions and densities of endothelin-A (ETA) and ETB receptor subtypes in peripheral lung alveolar wall tissue of the rat, guinea-pig and pig, with a view to assessing the potential suitability of these tissues as models for investigations of ET receptor function in human alveolar tissue. 2. High levels of specific [125I]-ET-1 binding were detected in peripheral lung components from all three species tested. In mature porcine alveolar wall tissue, specific binding increased in a time-dependent manner to a plateau, consistent with the previously described pseudo-irreversible binding of this ligand to a finite population of specific binding sites. 3. [125I]-ET-1 was associated specifically with both ETA and ETB binding site subtypes in alveolar wall tissue of foetal pig lung as early as 36 days gestation, raising the possibility of a functional role for ET-1 in lung development. In addition, both ETA and ETB binding site subtypes were detected in alveolar wall tissue and in peripheral airway smooth muscle of mature lung parenchyma from all three species. However, the binding subtype proportions differed in these tissues. For example, in porcine peripheral bronchial smooth muscle, ETA sites apparently predominated, whereas ETB sites constituted the major subtype detected in alveolar wall in this species. These data suggest significant shifts in ET receptor subtype expression at different levels in the respiratory tract. 4. ET binding site subtype proportions in the alveolar wall also differed markedly between species. In rat lung alveoli, ETA and ETB sites were detected in similar proportions (52 +/- 3% and 43 +/- 5% respectively). In contrast, in guinea-pig peripheral lung, ETB binding sites clearly predominated, constituting approximately 80% of total specific binding, with ETA sites accounting for only 12%. Porcine alveolar wall tissue also contained a mixture of these ET receptor subtypes, with ETA and ETB binding comprising 23 +/- 3% and 65 +/- 1% respectively of the total population of specific binding sites detected. These latter proportions are similar to values previously obtained in human peripheral lung tissue, suggesting that porcine lung might be a useful model of the human peripheral lung in subsequent studies of the functions of these pulmonary ET receptor subtypes.