The objective of this study was to evaluate a sustained release sucrose acetate isobutyrate (SAIB) in situ system formulation of risperidone (RSP) in vivo. The formulation contained SAIB, ethanol, and polylactic acid (PLA) as a release regulator. In vivo pharmacokinetics (PK) studies have shown that PLA is effective in reducing the burst effect. After a 12.5mg/kg IM injection of a 25mg/g RSP-SAIB in situ system, the C(max) was markedly reduced from 944.1+/-80.2 to 330.4+/-33.6ng/ml by increasing PLA from 1% to 10% (w/w), the T(max) were prolonged from 2 to 4.3+/-2.0h, and the area under the curve from day 0 to 2 (AUC(0-2day)) was reduced significantly from 16294.8+/-3946.4 to 7025.3+/-1979.2ngh/ml. For the RSP-SAIB in situ system including 10% PLA, the high release rates over a short period allowed therapeutic plasma concentrations to be achieved in the initial stages after activation, and sustained release of the drug led to a stable plasma concentration (by day 25, the plasma concentration was 8% of the C(max)). These initial in vivo studies suggest that RSP-SAIB in situ system is effective as a sustained delivery system.