1 In rabbit polymorphonuclear leukocytes (PMNLs) apomorphine at 10-100 microM inhibits fMet-Leu-Phe and A23187-induced exocytosis, and the phorbol myristate acetate- and fMet-Leu-Phe-induced activation of the metabolic burst. The secretory response was not restored by washing the cells after pretreatment with apomorphine. 2 The inhibitory effect of apomorphine was not prevented by the dopamine receptor antagonists haloperidol and pimozide, nor did dopamine itself inhibit fMet-Leu-Phe-induced exocytosis. It therefore seems unlikely that effects are mediated via dopamine receptors. However, sulphydryl reagents reduced the inhibitory effect of apomorphine, suggesting that it may depend upon interaction with susceptible sulphydryl groups, the intactness of which is required for exocytosis and other functions of PMNLs.