Recoveries of acepromazine and chlorpromazine from equine plasma were examined. Recoveries of both drugs from plasma were poor under theoretically optimal conditions for basic drugs. When a wide range of extraction pH was examined, it was found that more complete recoveries of these drugs from plasma were achieved at pH 5–6. Use of [3H] chlorpromazine showed that the rate of migration of the drug from an aqueous to a nonpolar environment was much faster at pH 6.0 than at pH 9.2 from both plasma and buffer solutions. Times required for equilibration with agitation were 15 min at pH 6.0, 1 h at pH 9.2, and 2 h at pH 11.0. With these agitation times and pH values, recoveries were more than 95% complete.