Leptin is a protein produced by adipose tissue that circulates to the brain and modulates metabolism, food intake, weight regulation and respiratory function. Leptin receptors and their mRNA have been identified in the solitary tract nucleus, specific region that is known to affect respiration (Mercer et al. 1998), suggesting a possible involvement of leptin in respiratory control. To test the hypothesis, respiratory effects of microinjections of leptin (10-10 - 10-4 M) into the solitary tract nucleus were characterized in anesthetized rats. Leptin in a concentration of 10-8 and 10-4 M induced a dose-dependent increase in respiratory minute volume mainly due to increased tidal volume. The responses were characterized by a respectively short latency (within 6 minutes); their duration depended on the concentration of leptin (up to 40 min. when 10-4 M leptin injected). 10-6 M leptin induced a biphasic response. During the initial inhibitory phase (approximately on 2nd min. after the microinjection) the tidal volume found to be reduced. The initial phase followed by a stimulatory phase (8th min. after the microinjection) when an increase in tidal volume was found. The changes in tidal volume induced by 10-6 M leptin accompanied by a transient increase in respiratory frequency (4-8 min. after microinjection) due to shortening of both inspiratory and expiratory duration. The effects of leptin on tidal volume usually correlated with changes in integrated activity of diaphragm and external intercostals muscles. The lowest concentration of leptin (10-10 M) appeared to be ineffective to induce any significant respiratory responses. The obtained data support the hypothesis about an involvement of leptin in respiratory control at the level of the solitary tract nucleus.