This is a double blind, randomized, placebo controlled, parallel group, flexible dose titration, add-on study. Male or female subjects aged 18-70 years suffering from Major Depressive Disorder according to DSM-IV, with a HAMD-17 score greater than 21 and a CGI-Severity of Illness score greater or equal to 4, will be started on open labeled citalopram treatment. The dose of citalopram may be increased form 20mg to 40mg over a six week period, depending on investigator assessment of tolerability and efficacy. At the end of this treatment, subjects with a HAMD-17 score greater or equal to 14 and a CGI-Severity of Illness score greater or equal to 4 will be considered as partial or non-responders and will be entered into the double blind phase of the study. Subjects will be randomized to either mecamylamine or placebo for a further 8 weeks. Citalopram medication will remain constant while mecamylamine (or placebo) can be increased from 5.0 to 7.5 to 10.0mg based on investigator assessment of tolerability and efficacy.