The effect of mucolytic and expectorant substances on ciliary beat frequency, mucus transport velocity and mucus production, was investigated in normal and bronchitic rats. The results showed that: (i) N-acetylcysteine and S-carboxymethylcysteine were midly cilioexcitatory at low and ciliodepressive at higher concentrations in both normal and bronchitic rats. A similar pattern was seen in mucus transport velocity, (ii) Bisolvon® enhanced all aspects of mucoeiliary activity in both groups of animals. Sobrepin was less effective than Bisolvon and more effective than Tachoquilin®. (iii) Geleomyrtol, Ozothin and prostaglandin E₁ were all cilioexcitatory in rats with bronchitis. Mucus transport velocity was similarly stimulated by both Geleomyrtol and Ozothin. (iv) Ammonium chloride and potassium iodide enhanced mucoeiliary activity in normal and bronchitic rats, (v) All substances stimulated mucus production, however, the most potent was prostaglandin E₁. The mechanisms for increased mucoeiliary activity involve inter alia the probable cleaving of disulfide bridges, decreased mucosal swelling, altered rheological characteristics and stimulation of adenylate cyclase.