  We study a massive scalar field theory in de Sitter space. Using worldline
instanton approach, we calculate probability of pair production in weak-field
limit. In addition to exponential factor we derive pre-exponential factor.
Within this approach the vanishing probability for odd-dimensional de Sitter
space gets a clear geometrical interpretation. We find leading contribution to
imaginary part of two point correlator in $\phi^3$-theory.
