The laminar distribution of tyrosine hydroxylase activity, dopamine and norepinephrine was determined in the dog olfactory bulb. The levels of tyrosine hydroxylase activity and dopamine were highest in the glomerular layer, whereas norepinephrine appeared to be more uniformly distributed across the layers. A similar distribution was observed within the mouse olfactory bulb. Following deafferentation of the mouse olfactory bulb, the levels of tyrosine hydroxylase activity and dopamine declined, while norepinephrine levels showed a transient increase. Subsequent to regeneration of the olfactory nerve, these levels returned to control values. The levels of tyrosine hydroxylase activity and of dopamine were very low or non-detectable in the olfactory epithelium, which contains the olfactory receptor neuron perikarya. The data obtained indicate that tyrosine hydroxylase activity and dopamine content in the bulb are more tightly coupled to each other than either is to norepinephrine content. Since the two catecholamines are in two different classes of neurons, this implies that the bulk of the tyrosine hydroxylase activity in the bulb is associated with the dopamine-containing neurons. Finally, our data are consistent with a transsynaptic control mechanism of the tyrosine hydroxylase activity and dopamine level in the olfactory bulb.