The effect of trifluoperazine (TFP) and phencyclidine (PCP) on acetylcholine receptor (AChR) function was studied in rat muscles differentiated in cell culture. While both drugs exerted an inhibitory effect on carbamylcholine (CCh)-induced Na+ or Ca2+ influx (I50 = 5-7 microM), alpha-bungarotoxin binding was not affected. The inhibitory effect of both drugs was independent of CCh concentration, which deems it unlikely that these drugs enhanced desensitization. The mutual inhibitory effect of TFP and PCP on Ca2+ influx was analyzed using three alternative models of interaction between the two drugs: competitive, additive and synergistic inhibition models. Our results are in accordance with a synergistic interaction between the drugs. This synergistic interaction between the drugs provides a biochemical rationale to the phenothiazine contraindication in the treatment of PCP psychosis.