The formation of 3-(4-hydroxy-1-naphthoxy)lactic acid (4-HO-NLA) from propranolol was investigated. Authentic 4-HO-NLA was synthesized from 4-methoxy-1-naphthol using methods previously used for preparation of naphthoxylactic acid (NLA). Cleavage of the 4-methyl ether was accomplished using iodotrimethylsilane in the presence of cyclopentene. After ip administration of propranolol-3,3-d2 (P-3,3-d2) (20 mg/kg) to rats, 4-HO-NLA-d2 was identified by GC-MS as a urinary metabolite. After administering pseudoracemic propranolol (S-P-d0/R-P-3,3-d2) ip to rats (20 mg/kg), parent ions of 4-HO-NLA-d0 and -d2 as methyl ester-trifluoroacetyl derivatives were monitored by GC-MS. 4-HO-NLA arose stereoselectively from S-propranolol (S/R ratio 2.6). After administering approximately equimolar quantities of 4-HO-P and P-3,3-d2 (10 mg/kg each, ip), only 4-HO-NLA-d2 arising from metabolism of P-3,3-d2 was observed by GC-MS indicating that 4-HO-P is probably not an important metabolic intermediate in vivo. In vitro experiments in the presence of the rat liver supernatant fraction performed with P-3,3-d2, NLA, and 4-HO-P showed that only NLA led to 4-HO-NLA. Incubation of NLA also produced two other hydroxylated NLA regioisomers. Incubation of 4-methoxypropanolol, a more lipophilic congener of 4-HO-P, produced no lactic acid metabolite in the presence of the rat liver supernatant fraction, indicating that poor lipophilicity is not the only deterrent to N-dealkylation of the side chain of 4-HO-P.(ABSTRACT TRUNCATED AT 250 WORDS)