Noradrenergic properties of peripheral sympathetic neurons obtained from 10-12-day-old chick embryos were examined under various culture conditions. Sympathetic neurons supported by nerve growth factor in serum-free or serum-containing medium took up significant and almost equivalent amounts of [3H]norepinephrine. The uptake was markedly enhanced when neurons were co-cultured with heart cells, either in the absence or presence of nerve growth factor, for 3 days. The facilitatory effect of heart cells on the uptake was persistent only if the nerve growth factor was present. In its absence there was a gradual decrease in the uptake. Endogenous norepinephrine content was increased by several fold when sympathetic neurons were grown with either heart cells or in a medium conditioned by the heart cells. Sympathetic neurons initially selected in culture by nerve growth factor in regular medium and then exposed to a conditioned medium for 3 days exhibited a marked facilitation of [3H]norepinephrine uptake. The number of surviving neurons was almost constant when culture media were changed. Choline acetyltransferase activity of neurons grown in heart-conditioned medium plus nerve growth factor was not significantly higher than that of neurons grown in regular medium plus nerve growth factor. The overall conclusion of the study is that the noradrenergic character of sympathetic neurons can be further enhanced by heart cells or a medium conditioned by these cells.