The effects on self-stimulation behaviour of 5 mug morphine HCl applied into the ventricular system and into different areas throughout the brain were studied. Injections into the ventricular system and in areas intermediate between the posterior hypothalamus and the periaqueductal grey matter had biphasic effects: an inhibition followed by an excitation. Injections into the posterior hypothalamus resulted in increased self-stimulation whereas injections into the periaqueductal grey matter and into the locus coeruleus were only inhibiting.