The tachyphylaxis to DSCG’s inhibition of anaphylactic histamine release has been demonstrated in passively sensitized rat lung fragments. The induction of tachyphylaxis appears to depend on the concentration of the drug and the length of pretreatment. Tachyphylaxis is relatively independent of the concentration of the second exposure to DSCG. The development of tachyphylaxis is highly temperature dependent; it can be prevented by cooling the tissues to 2–4°C after a brief (30 sec) preincubation with DSCG. It is suggested that DSCG inhibits histamine release by binding to ‘receptor’ site(s). Once the site is occupied by DSCG, it is modified by a temperature-dependent process, thus losing the ability as a ‘receptor’ for inhibition of histamine release. The tachyphylaxis is the result of such a modification.