The pharmacological effects of N-[4-(2-bromoethylmethylamino)-2-butynyl]-2-pyrrolidone (BR 401) were compared in the mouse with those of N-[4-(2-chloroethylmethylamino)-2-butynl]-2-pyrrolidone (BM 123) and oxotremorine. BR 401 was more toxic than oxotremorine and BM 123 when administered i.v. (LD50, 0.7 mumol kg-1), but less toxic than oxotremorine when given i.p. (LD50, 39 mumol kg-1). Atropine and methylatropine (10 mumol kg-1 i.p.) increased the LD50 value of BR 401, given i.v., 75- to 100-fold. Upon i.v. administration, BR 401 was 2- to 3-fold more potent than oxotremorine and 10 to 20 times more potent than BM 123 in producing central (tremor and analgesia) and peripheral (salivation) muscarinic effects. However, after i.p. administration BR 401 was 3-fold less potent than oxotremorine in eliciting tremor and analgesia. The aziridinium ion (BR 401A), formed by cyclization of BR 401, produced salivation but no tremor. These observations suggest that BR 401 when given i.v. penetrates effectively into the central nervous system where it cyclizes rapidly to the pharmacologically active aziridinium ion. In contrast, after i.p. administration a large proportion of BR 401 will cyclize before it can reach the central nervous system. BM 123, because of its slower cyclization, enters the central nervous system effectively also by the i.p. route. Thus, central potency of 2-haloethylamines such as BR 401 and BM 123 is critically dependent not only on the rate of cyclization, but also on the route of administration. The duration of tremor induced by BR 401 and BM 123 was considerably shorter than that induced by oxotremorine.(ABSTRACT TRUNCATED AT 250 WORDS)