Effects of histamine, histamine agonists and antagonists on steady state current in principal neurons and interneurons were investigated in thin slices from the olfactory bulb of newborn rabbits with the nystatin perforated patch-clamp technique and local pipette application. No change in steady state current was observed in mitral cells. In most of the periglomerular, juxtaglomerular and granular cells, however, H1-receptor activation caused an outward current; a similar effect, but mostly not on the same neurons was elicited by 8-bromo-cyclic-AMP. These currents were reversed at the potassium equilibrium potential, indicating block of a potassium current. Specific H3-receptor activation and cyclic GMP were ineffective. Histamine usually caused a combined effect beginning with an inward current. Histaminergic neurons fire with changes in behavioural state and can, by the described mechanisms, markedly influence signal processing in the olfactory bulb.