N-5' shows a potent inhibitory action on the homologous passive cutaneous anaphylaxis (PCA) in rats mainly through the inhibition of histamine release from mast cells. The present experiment was an attempt to clarify in detail the pharmacological properties of N-5'. Inhibition of PCA was most potent at 30 or 60 min pretreatment with N-5', and negligible at 240 min pretreatment. Given p.o., N-5' produced a dose-dependent, potent inhibitory action at 30-min pretreatment. On the other hand, disodium cromoglycate (DSCG) had little effect on PCA when given orally. On the case of i.v. administration, N-5' (20 mg/kg) and DSCG (5 mg/kg) showed a most potent inhibition of PCA at 5 min pretreatment. The inhibitory action of DSCG was, however, shorter lasting than that of N-5'. Median effective doses (ED50) of DSCG and N-5' on the PCA were estimated to be 0.79 and 8.8 mg/kg i.v., respectively. Inhibitory activity of N-5' in the adrenalectomized rat did not differ from that in sham operated animals. N-5' had a more potent inhibitory action on the PCA in infant rats than in adults. Inhibitory activity of N-5' in the case of 1, 2, 3 and 4 weeks of successive administration was equipotent to that with a single administration.