We determined the affinities of nordimaprit, homodimaprit, clobenpropit and imetit for H3 binding sites (labelled by 3H-N alpha-methylhistamine) in rat brain cortex homogenates and their potencies at presynaptic H3A receptors on noradrenergic nerve endings in mouse brain cortex slices. 3H-N alpha-Methylhistamine bound saturably to rat brain cortex homogenates with a Kd of 0.70 nmol/l and a Bmax of 98 fmol/mg protein. Binding of 3H-N alpha-methylhistamine was displaced monophasically by dimaprit (pKi 6.55), nordimaprit (5.94), homodimaprit (6.44), clobenpropit (9.16), imetit (9.83), R-(-)-alpha-methylhistamine (8.87) and histamine (8.20), and biphasically by burimamide (pKi high 7.73, pKi low 5.97). In superfused mouse brain cortex slices preincubated with 3H-noradrenaline, the electrically (0.3 Hz) evoked tritium overflow was inhibited by imetit (pIC35 8.93), R-(-)-alpha-methylhistamine (7.87) and histamine (7.03). The effect of histamine was attenuated by nordimaprit, homodimaprit, clobenpropit and N-ethoxycarbonyl-2- ethoxy-1,2-dihydroquinoline (EEDQ); EEDQ (but not nordimaprit, homodimaprit and clobenpropit) attenuated the effect of histamine also in slices pre-exposed to the drug 60-30 min prior to superfusion. The concentration-response curve of histamine was shifted to the right by homodimaprit and clobenpropit; Schild plots yielded straight lines with a slope of unity for both drugs (pA2 5.94 and 9.55, respectively). Nordimaprit depressed the maximum effect of histamine (pD'2 5.55) and also slightly increased the concentration of histamine producing the half-maximum effect.(ABSTRACT TRUNCATED AT 250 WORDS)