Precise pharmacokinetic data of long-acting neuroleptics: apparent half life (T 1/2), time of peak plasma concentration (Tmax), bioavailability, has been a major contribution to determine optimal dosage of the drug. If the aim of the depot neuroleptic is to obtain a stable plasma concentration of the neuroleptic after I.M. injection of the ester form equivalent to that following oral administration, it is logical to obtain the same pharmacological effect; this is true for haloperidol decanoate. Mean value of T 1/2 of clopenthixol decanoate and haloperidol decanoate are 19 and 21 days, respectively, they thereby justify monthly administration. Flupenthixol decanoate and fluphenazine enanthate should be injected with dosing intervals of 3 and 1 weeks, respectively in respect with their half-lives: 17 and 4 days. Fluphenazine decanoate have a half-life of 14 days, however, the longer time the treatment, the longer the apparent half-life, suggesting to reduce the dose or to enlarge the dosing interval. Optimal dose has been determined from the bioavailability of the oral formulation and the interval between two injections, it averages 15, 20 times the oral daily dose for haloperidol decanoate. A lower conversion factor is frequently used (0.5 to 5 times) for other depot-neuroleptics such as pipotiazine palmitate, fluphenazine enanthate or decanoate; these low factors are not entirely explainable by the low bioavailability of the oral forms and produces more lower plasma concentration than after oral administration.