Two double blind crossover studies have been carried out in healthy volunteers to determine the effect on platelet behaviour and on blood coagulation of treatment with atenolol and with a fixed combination of atenolol and nifedipine. The first study compared atenolol with placebo, whilst the second compared atenolol alone with the combination of atenolol and nifedipine. Treatment with atenolol alone had no effect on platelet behaviour. In contrast, treatment with the combination of the beta blocker and the calcium antagonist gave rise to a small but significant increase in platelet dense granule release ex vivo in response to a wide range of agonists. This was not associated with any change in serum levels of thromboxane B2 nor with any alteration in plasma concentrations of platelet factor 4 and beta-thromboglobulin. Neither treatment had any effects on coagulation parameters. The mechanism and clinical significance of the observed increase in the extent of the platelet release reaction during combined atenolol/nifedipine treatment remain speculative.