1. The response to perivascular nervous stimulation (PNS) and the responsiveness to receptor agonists, in different stages of neurogenesis, on rat mesenteric vascular bed (MVB), was investigated. Rats of different age groups (5-7, 9-11, 14-16, 20-22 days) were tested, using 60-day-old rats as controls. 2. In the 5-7 days age group, the response to PNS was resistant to TTX treatment (1 x 10(-6) M). The TTX inhibition increased with age and became almost complete in 60-day-old rats. 3. In the 1st week of postnatal life (pre-innervation period), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) produced contraction, whereas isoprenaline (ISO) and dopamine (DA) caused relaxation. During the 1st and 2nd week, pD2 values of NA and ISO were significantly higher than in adult control rats. No significant difference in pD2 values of 5-HT and DA was observed during postnatal development. 4. At 5-7 days, the relaxation by acetylcholine (ACh), typical of adult age, was absent and ACh evoked only contractile responses. The relaxant effect by ACh appeared at 9-11 days, increased with age and, by the end of the 2nd week, did not differ from that of the adult group. 5. These results provide evidence that responsiveness of all tested receptors in the MVB is already present in the pre-innervation period (1st week). Adrenergic receptor responsiveness is higher at birth and decreases by the end of the 3rd week of postnatal life, when connections between the central nervous system and effector organs are established. Only muscular muscarinic receptors, responsible for ACh-induced contraction, are functional in the 1st week, while endothelial muscarinic receptors, responsible for ACh-induced relaxation, become gradually responsive later in postnatal life.