Middlesex have signed all-rounder Ollie Rayner from Sussex on a three-year deal after he impressed during two loan spells with the side over the summer.
Rayner, 25, hit 266 runs and took 11 wickets in eight County Championship games for Middlesex during 2011.
Director of cricket Angus Fraser said: "Ollie is an exciting and talented cricketer who contributes with bat, ball and in the field."
Meanwhile, Jamie Dalrymple has left the county after a second spell at Lord's.
The former England all-rounder re-signed for Middlesex in May and went on to take 10 wickets and hit 505 runs at an average of 36 in 11 County Championship games during the 2011 season.
Dalrymple, 30, had previously left Lord's in 2007 after making 2,725 runs at 30.27 in 62 first-class matches, to join Glamorgan.
But he resigned from Glamorgan in 2010 after being replaced as captain, and later rejoined Middlesex.
Fraser added: "Jamie's contribution on and off the field was invaluable.
"With the bat he scored vital runs at crucial times and he also claimed important wickets.
"However, his passion lies in leadership and man management roles and these are not positions that are currently available at Middlesex."
Dalrymple, who played 27 one-day internationals and three Twenty20 games for England, continued: "I now have a chance to consider whether I want to keep playing, to look for a leadership role within the game or to move on to something completely different."
Rayner, who took 95 wickets and scored over 1,000 runs in 43 first-class matches during six years at Sussex, is looking forward to helping his new side to further success, after helping to secure promotion back to the County Championship top flight.
"It was a great privilege to be involved in the Division Two championship-winning side," he explained. "To date it has been a highlight of my career and long may the success continue."
Fraser added: "On several occasions last summer he showed exactly what he is capable of producing. Ollie has presence on a cricket field and works extremely hard at his game."
Meanwhile, Middlesex have handed new contracts to two of their young prospects.
Left arm seamer Gurjit Sandhu, who made his first-class debut for Middlesex against Sri Lanka at Uxbridge this summer, has signed a two-year deal.
And left-hander Adam London marks his 23rd birthday this week by signing a new one-year deal, which will keep him at Lord's at least until the end of the domestic 2012 season.
"This time last year Adam was recovering from a serious knee operation," added Fraser. "It took him a fair part of last summer for his rehabilitation to be completed, but te finished the 2011 season strongly in the second XI."