Given a fixed acute angle $\theta$ and a pair of internally tangent circles, let the line $l$ which passes through the point of tangency, $A$, cut the larger circle again at $B$ ($l$ does not pass through the centers of the circles).  Let $M$ be a point on the major arc $AB$ of the larger circle, $N$ the point where $AM$ intersects the smaller circle, and $P$ the point on ray $MB$ such that $\angle MPN = \theta$.  Find the locus of $P$ as $M$ moves on major arc $AB$ of the larger circle.
1. **Identify the given elements and their relationships:**
   - We have two internally tangent circles with a common tangency point \( A \).
   - A line \( l \) passes through \( A \) and intersects the larger circle again at \( B \).
   - \( M \) is a point on the major arc \( AB \) of the larger circle.
   - \( N \) is the point where the line segment \( AM \) intersects the smaller circle.
   - \( P \) is a point on the ray \( MB \) such that \( \angle MPN = \theta \).

2. **Analyze the triangles \( MPN \) and \( MPA \):**
   - Since \( \angle MPN = \theta \) is constant, and \( \angle PMN \) is also constant (as it is determined by the fixed angle \( \theta \) and the geometry of the circles), all triangles \( MPN \) are similar to each other by AA similarity criterion.
   - The ratio \( \frac{AM}{AN} \) is constant because \( A \) and \( N \) are fixed points on the smaller circle, and \( M \) is moving along the larger circle. This implies that the triangles \( MPA \) are also similar to each other.

3. **Determine the transformation involved:**
   - The similarity of triangles \( MPA \) implies that there is a spiral similarity \( \mathcal{S} \) centered at \( A \) that maps the larger circle to another circle.
   - A spiral similarity is a combination of a rotation and a dilation centered at a point. Here, the center is \( A \), and the transformation maps the major arc \( AB \) of the larger circle to another arc.

4. **Find the locus of \( P \):**
   - Since \( P \) is determined by the spiral similarity \( \mathcal{S} \) applied to the points on the major arc \( AB \), the locus of \( P \) is the image of the major arc \( AB \) under this spiral similarity.
   - Therefore, \( P \) moves on a circle which is the image of the larger circle through the spiral similarity \( \mathcal{S} \) centered at \( A \).

Conclusion:
The locus of \( P \) as \( M \) moves on the major arc \( AB \) of the larger circle is a circle, which is the image of the larger circle through a spiral similarity centered at \( A \).

The final answer is the locus of \( \boxed{ P } \) is a circle.