Objective: To investigate the potential for systemic pharmacodynamic and pharmacokinetic interactions between inhaled salmeterol and fluticasone propionate when repeat doses of the two drugs are given in combination to healthy subjects. Methods: Twenty-eight healthy subjects received salmeterol 100 µg, salmeterol 100 µg/fluticasone propionate 500 µg and fluticasone propionate 500 µg via a Diskus dry powder inhaler twice daily for 11 days according to a randomised, double-blind, placebo-controlled, crossover design. Subjects in the placebo group also received a single dose of salmeterol 100 µg on the morning of day 10. On day 10, the systemic effects of salmeterol [on pulse rate, blood pressure, corrected QT (QTc) interval and serum potassium and glucose levels] and fluticasone propionate (on 24-h urinary cortisol and morning plasma cortisol levels) were assessed. Maximal number and affinity of lymphocyte β2-adrenoceptors and β2-adrenoceptor polymorphism at loci 16 and 27 were also determined. Plasma pharmacokinetics of salmeterol and fluticasone propionate were determined after the morning dose on day 10. Dosing continued on the evening of day 10 and on day 11, and on day 12 the effect of repeat-dose treatment with salmeterol and salmeterol/fluticasone propionate on the systemic effects of cumulative doses of inhaled salbutamol (up to a total dosage of 3200 µg) was evaluated. Results: All treatments were safe and well tolerated. With the exception of a higher pulse rate after repeat administration of salmeterol [66.2 beats per minute (bpm) versus 63.6 bpm], there were no significant differences between the single-dose and repeat-dose salmeterol groups. The systemic pharmacodynamic effects of inhaled salmeterol were not affected by the co-administration of fluticasone propionate. Eleven days of treatment with salmeterol induced tachyphylaxis to the systemic effects of cumulative doses of salbutamol; however, co-administration of fluticasone propionate did not affect the response to salbutamol. Fluticasone propionate reduced 24-h urinary cortisol excretion (22.4 µg compared with 48.6 µg with placebo), but this was unaffected by the co-administration of salmeterol. Morning plasma cortisol levels were not reduced compared with placebo. There was no significant treatment effect on lymphocyte β2-adrenoceptors and no correlation of β2-adrenoceptor polymorphism at loci 16 and 27 with the development of tachyphylaxis. Salmeterol plasma concentrations were measurable only during the first half-hour after dosing. Co-administration of fluticasone propionate did not affect the peak plasma concentration (Cmax) of salmeterol. For fluticasone propionate, there were no statistically significant differences between salmeterol/fluticasone propionate and fluticasone propionate with respect to Cmax, plasma concentration at the end of the dosing interval (Ct), terminal elimination half-life (t1/2) or time to Cmax (tmax). The area under the concentration–time curve within a dosing interval (AUCt) for fluticasone propionate after inhalation of salmeterol/fluticasone propionate was statistically significantly higher (about 8%) than after inhalation of fluticasone propionate alone (P=0.0135). However, the 90% confidence intervals (CIs) for the AUCt and Cmax ratios for the two treatments were within the accepted limits for bioequivalence (1.03, 1.13 and 0.97, 1.12, respectively). Conclusion: These results in healthy subjects indicate that there is no systemic pharmacodynamic or pharmacokinetic interaction between inhaled salmeterol and fluticasone propionate when given in combination.