The development of heart rate and of muscarinic and beta-adrenergic receptors in chick heart ventricles in the last third of in ovo ontogenesis has been studied. The development of these two types of autonomic receptors does not proceed in parallel. While muscarinic receptors are stable between days 13 and 17, beta-adrenergic receptors significantly decrease in this period. Muscarinic receptors increase their number from the 17th day, and the highest density can be seen after hatching. beta-adrenergic receptors, on the contrary, do not change their density between days 17 and 19, but similarly to muscarinic receptors they increase after hatching (the density after hatching is approximately the same as on the day 13 of embryonic development). We have shown previously that stimulation of one receptor type in the system of autonomic receptors in the heart changes not only the appropriate type of receptor but also the density of the other one. We therefore wondered the information about this in the organism in development. Here we show that when we have infused the eggs in developmental period mentioned above there was no cross-regulation and that muscarinic receptors do not down-regulate. Up-regulation of muscarinic receptors was maintained when the eggs were infused by carbachol from the 13th to 14th day of in ovo development. This muscarinic receptor up-regulation was accompanied with paradoxical increase in heart rate. These events are not proteinkinase (PKC) dependent, as bisindolylmaleimide (PKC inhibitor) do not prevent these changes.