1. The non-hydrolysable cyclic AMP analogue, dibutyryl (Bu2)-cyclic AMP, inhibited the stimulated release of histamine from both basophils and human lung mast cells (HLMC) in a dose-dependent manner. The concentrations required to inhibit histamine release by 50% (IC50) were 0.8 and 0.7 mM in basophils and HLMC, respectively. The cyclic GMP analogue, Bu2-cyclic GMP, was ineffective as an inhibitor of histamine release in basophils and HLMC. 2. The non-selective phosphodiesterase (PDE) inhibitors, theophylline and isobutyl-methylxanthine (IBMX) inhibited the IgE-mediated release of histamine from both human basophils and HLMC in a dose-dependent fashion. IBMX and theophylline were more potent inhibitors in basophils than HLMC. IC50 values for the inhibition of histamine release were, 0.05 and 0.2 mM for IBMX and theophylline, respectively, in basophils and 0.25 and 1.2 mM for IBMX and theophylline in HLMC. 3. The PDE 4 inhibitor, rolipram, attenuated the release of both histamine and the generation of sulphopeptidoleukotrienes (sLT) from activated basophils at sub-micromolar concentrations but was ineffective at inhibiting the release of histamine and the generation of both sLT and prostaglandin D2 (PGD2) in HLMC. Additional PDE 4 inhibitors, denbufylline, Ro 20-1724, RP 73401 and nitraquazone, were all found to be effective inhibitors of mediator release in basophils but were ineffective in HLMC unless high concentrations (1 mM) were employed. 4. Neither 8-methoxymethyl IBMX (PDE 1 inhibitor), zaprinast (PDE 5 inhibitor) nor a range of PDE 3 inhibitors (siguazodan, SKF 94120, SKF 95654) were effective inhibitors of mediator release from either basophils or HLMC. 5. In basophils, rolipram acted to potentiate the inhibitory effects of the adenylate cyclase activator, forskolin, whereas in HLMC, rolipram failed to potentiate the inhibitory effects of forskolin. 6. Extracts of purified HLMC and basophils hydrolysed cyclic AMP. IBMX (100 microM) inhibited the PDE activity in basophil extracts by 67 +/- 7% (P < 0.0001) and in HLMC extracts by 63 +/- 9% (P < 0.0005). The hydrolysis of cyclic AMP by basophil extracts was inhibited by the selective PDE inhibitors (all at 10 microM), rolipram (56 +/- 8%, P < 0.0001) and the mixed PDE 3/4 inhibitor, Org 30029 (47 +/- 9%, P < 0.01), whereas 8-methoxymethyl IBMX, siguazodan and zaprinast were ineffective. In HLMC, rolipram, Org 30029, 8-methoxymethyl IBMX, siguazodan and zaprinast all inhibited the hydrolysis of cyclic AMP by extracts to a significant (P < 0.05) and similar extent (approximately 25% inhibition at 10 microM). 7. In total, these data suggest that modulation of the PDE 4 isoform can regulate basophil responses whereas an association of the PDE 4 isoform with the regulation of HLMC function remains uncertain.