it turned up the heat again last week, although Robert C. McFarlane, the White House national security adviser, reiterated that there were no plans for an invasion or even for a break in diplomatic relations. In a diplomatic note, which Nicaragua rejected as containing ''false accusations,'' the United States said intelligence reports showed Nicaragua supported people planning terrorist attacks on Americans in Honduras, and that the Sandinistas also supported Salvadoran insurgents who killed four off-duty marines and two other Americans last month in El Salvador. Repetition anywhere in Central America would have ''serious consequences,'' the note warned. ''Our response to violent acts against Americans will be appropriate to the loss incurred,'' the White House said. ''The time and place will be of our own choosing.'' Invoking for the first time Congressional authority approved in 1984, the State Department offered a $100,000 reward for information leading to punishment of the assassins. A U.S. WARNING TO NICARAGUA

It Turns From Stabilization to Foreign Outlets and Develop- ing of Cooperatives. STONE HITS SPECULATORS Chairman Says They Have Tried to Balk Aid to Wheat and Cotton Producers.

it turns out our existence may change the brains of the animals around us; a group of researchers is bringing water quality tracking to the taps we drink from. An Enormous I.V. Markup, Humans as Evolutionary Force, Guerrilla Water Sampling

it turns out that 'bomb' was something less than that, and prosecutors acknowledge that boys were not out do hurt anyone; Mallner is confined to his home by court order as punishment for breaking into school; photos (M) The Little Terror Who Wasn't; No Bomb, but Expelled Boy Still Finds Zero Tolerance

it turns out that families try to save food by executing less-productive elderly women as witches. Freakonomics Developnomics

It turns out that Johnny Miller is human after all. Miller's 71 5 Strokes Off Pace