The effects of lysergic acid diethylamide (LSD), d-amphetamine (AMP), chlorpromazine (CPZ), and the most active isomer of marihuana (delta9 - THC) on timing behavior were analyzed with a two-choice, discrete trial procedure in which pigeons were trained to discriminate visual stimuli that differed with respect to duration ('long' vs. 'short'). LSD (0.01, 0.04, 0.16 mg/kg) decreased response speed (increased latency), but otherwise had no significant effects on performance of the discrimination. d-Amphetamine (1.0, 2.0, 4.0 mg/kg) increased perseveration of 'spatial bias' and, at a dose of 4.0 mg/kg, lowered response speed. This compound did not significantly alter accuracy (percentage correct). CPZ (7.5, 15.0, 30.0 mg/kg) significantly decreased accuracy and, at a dose of 30.0 mg/kg, significantly lowered speed; THC also decreased accuracy and lowered speed. Neither CPZ nor THC significantly altered perseveration.