The postnatal development of monoamine levels and receptors in the occipital cortex of the cat has been investigated using neurochemical techniques. The endogenous catecholamines (noradrenaline and dopamine) gradually increased with age, displaying an about 12-13-fold increase in their concentration from the newborn to the adult stage. 3H-dihydroalprendol (beta-adrenoceptor ligand) binding showed a rapid increase from the low value (25% of the adult value) at birth, peaking at the age of 7-9 weeks with a value of about 150% of adults. The beta-adrenoceptor binding stayed relatively constant at adult value from the age of 11 weeks throughout. Endogenous 5-hydroxytryptamine levels were at birth about 20% of the adult value and thereafter rapidly increased, peaking at the age of 3-5 weeks when it reached the adult value. Between the age of 7-13 weeks the 5-hydroxytryptamine level was about 50-60% of adult. The developmental pattern for 3H-5-hydroxytryptamine binding was similar to that of endogenous 5-hydroxytryptamine, although with certain quantitative differences. The 3H-5-hydroxytryptamine receptor binding showed a steep peak at an age of about 4 weeks when the binding was about 300% of the adult value. Thereafter the binding gradually levelled off in adulthood. Similar results were obtained in the frontal cortex, except for some quantitative differences. The present results thus indicate that both noradrenaline and 5-hydroxytryptamine nerve terminals develop, largely independent of their postsynaptic receptors, probably due to different developmental programs regulating their expression. The development of monoamine receptors appear to precede that of their nerve terminals. The different roles played by beta-adrenoceptors and 5-hydroxytryptamine receptors for the maturation of occipital cortex during postnatal critical period were discussed.