Acadia Pharmaceuticals, Inc. 
(Name of Issuer)

Common Stock 

(Title of Class of Securities)

004225108 
(CUSIP Number)

12/31/2008 
(Date of Event Which Requires Filing of this Statement)

[X]

Rule 13d-1(b)

[ ]

Rule 13d-1(c)

[ ]

Rule 13d-1(d)

CUSIP No. 004225108

1.

Names of Reporting Persons:
 
OppenheimerFunds, Inc.

2.

Check the Appropriate Box if a Member of a Group (See Instructions): Joint filing

3.

SEC Use Only

4.

Citizenship or Place of Organization:
Colorado

Number of Shares Beneficially Owned by Each Reporting Person With:

5.

Sole Voting Power:

0

6.

Shared Voting Power:

2,260,600

7.

Sole Dispositive Power:

0

8.

Shared Dispositive Power:

2,260,600

9.

Aggregate Amount Beneficially Owned by Each Reporting Person:

2,260,600 (beneficial ownership disclaimed pursuant to Rule 13d-4 of the Exchange Act of 1934)

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)
[ ]

11.

Percent of Class Represented by Amount in Row (9):

6.09%

12.

Type of Reporting Person (See Instructions):
IA

Item:

1(a)

Name of Issuer:

Acadia Pharmaceuticals, Inc.

1(b)

Address of Issuer's Principal Executive Offices:

3911 Sorrento Valley Blvd.

San Diego, CA 92121

2(a)

Name of Person Filing:

OppenheimerFunds, Inc.

2(b)

Address of Principal Business Office or, if none, Residence:

Two World Financial Center

225 Liberty Street
New York, NY 10281

2(c)

Citizenship:

Colorado

2(d)

Title of Class of Securities:
Common Stock

2(e)

CUSIP Number:

004225108

3

OppenheimerFunds, Inc. is an investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E).

4(a)

Amount beneficially owned:

2,260,600 (beneficial ownership disclaimed pursuant to Rule 13d-4 of the Exchange Act of 1934)

4(b)

Percent of class:

6.09%

4(c)
 

(i)

(ii)

(iii)

(iv)

Number of shares as to which the person has:
 

Sole power to vote or to direct the vote:

0

Shared power to vote or to direct the vote:

2,260,600

Sole power to dispose or to direct the disposition of:

0

Shared power to dispose or to direct the disposition of:

2,260,600

5.

Ownership of Five Percent or Less of a Class: [ ]

6.

Ownership of More than Five Percent on Behalf of Another Person.:

N/A

7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company:
N/A

8.

Identification and Classification of Members of the Group:
N/A

9.

Notice of Dissolution of Group:
N/A

10.

Certification:
By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.