This is a four part, parallel group, randomised, study to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamic (PD) effects of repeat doses of GSK1018921 in healthy volunteers and stable patients with schizophrenia and to evaluate its effect on pharmacokinetics of midazolam. Part A will evaluate 14 days repeat BID dosing in at least three cohorts of healthy volunteers, to assess safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of GSK1018921. Part B will study the potential drug interaction of GSK1018921 (Maximum Tolerated Dose from Part A) with midazolam with 14 days repeat BID dosing in healthy volunteers and therefore will assess the PK, safety and tolerability. Part C will be a single dose study in healthy volunteers and will include CSF sampling to assess the concentration of GSK1018921 and glycine in CSF with two doses (80 and 200 mg). Part D will study stable patients with schizophrenia, to assess safety, tolerability, PK and PD following 28 days of repeat BID dosing.