Unocal Corp. said yesterday it plans to hold its annual shareholders meeting April 29 despite a request for a delay from Mesa Petroleum Co. Chairman T. Boone Pickens Jr. while he decides whether to try to get control of Unocal.

Pickens, who heads a group that owns 13.6 percent of Unocal's stock, said he was nevertheless soliciting shareholder proxy votes to postpone the meeting until June 28. Unocal, until last year known as Union Oil Co. of California, has held its annual meeting on the last Monday in April for decades.

"Unocal does not intend to inconvenience its shareholders just because T. Boone Pickens wants a delay," Unocal Chairman Fred L. Hartley said in a statement.

"The shareholders are going to decide that," Pickens retorted in a telephone interview from Mesa's Amarillo, Tex., headquarters.

The sharp exchange could be a precursor of what many analysts believe could be a spectacular battle for control of Unocal between two of the oil industry's most colorful and outspoken executives. Pickens has been involved in pitched battles for control of Gulf Co., Cities Service Co. and, most recently, Phillips Petroleum Co. In all three cases, as in most of Pickens' other offers for oil companies, Pickens and his partners have not succeeded but have profited handsomely.