Caffeine (C) decreases apneic episodes in premature infants and is thought to stimulate breathing mainly by a central mechanism. While the methylxanthines theophylline and aminophylline are known to alter the carotid chemoreceptor activity, there are little data on C. The aim of the study was to examine the effects of C on the carotid sinus nerve discharge (CSND) in developing animals. Nine kittens 17-21 days old and six adult cats that were anesthetized and artificially ventilated were studied. They received four consecutive doses of C, each of 10 mg/kg, administered at intervals of 20 min either as intravenous bolus injection (6 kittens, 3 cats) or continuous infusion (3 kittens, 3 cats). Bolus injections of C invariably induced a prompt but transient increase in the CSND from 4.1 +/- 0.6 to 8.1 +/- 1.0 (SE) impulses/s in kittens (P = 0.01) and form 3.9 +/- 0.1 to 7.9 to 1.0 impulses/s in cats (after the first injection). This response was associated with a significant decrease in arterial blood pressure. Continuous infusion of C did not induce any early change in either CSND or blood pressure in kittens or cats. Fifteen minutes after C injection or infusion was begun, CSND values in air, 8% O2-balance N2, or 100% O2 were not significantly different from control. Haloperidol administered at the end of the experiment in four cats and four kittens significantly increased CSND and did not suppress the early response to C injection. It is concluded that caffeine administered by bolus in the kitten induces a transient stimulation of the CSND that is associated with a decrease in the arterial blood pressure and is independent of the dopaminergic mechanisms in the carotid body. The lack of sustained effect implies the main mechanism to the ventilatory stimulation by C must be central.